In the UK’s first wave, more than one in ten COVID hospitalised patients acquired the disease in a hospital according to researchers conducting the world’s largest study of severe COVID.
Dr Jonathan Read from Lancaster University with colleagues from other UK universities led the research into hospital-acquired infections (HAIs) which was published in The Lancet.
For the study, researchers analysed records of COVID patients in UK hospitals enrolled in the International Severe Acute Respiratory and Emerging Infections Consortium (ISARIC) Clinical Characterisation Protocol UK (CCP-UK) study, who became ill before 1st August 2020.
The researchers found that at least 11.1% of COVID patients in 314 UK hospitals were infected after admission. The proportion of hospital-acquired infections also rose to between 16% and 20% in mid-May 2020, well after the first wave’s peak in admissions.
“We estimate between 5699 and 11 862 patients admitted in the first wave were infected during their stay in hospital. This is, unfortunately, likely to be an underestimate, as we did not include patients who may have been infected but discharged before they could be diagnosed,” the researchers said.
“Controlling viruses like SARS-CoV-2 has been difficult in the past, so the situation could have been much worse. However, infection control should remain a priority in hospitals and care facilities,” said Dr Read.
Dr Chris Green, University of Birmingham, said: “There are likely to be a number of reasons why many patients were infected in these care settings. These include the large numbers of patients admitted to hospitals with limited facilities for case isolation, limited access to rapid and reliable diagnostic testing in the early stages of the outbreak, the challenges around access to and best use of PPE, our understanding of when patients are most infectious in their illness, some misclassification of cases due to presentation with atypical symptoms, and an under-appreciation of the role of airborne transmission.”
According to the type of care provided, there were notable differences in infections. Lower proportions of hospital-acquired infection were seen in hospitals providing acute and general care (9.7%) than residential community care hospitals (61.9%) and mental health hospitals (67.5%). Professor Calum Semple, University of Liverpool, said: “The reasons for the variation between settings that provide the same type of care requires urgent investigation to identify and promote best infection control practice. Research has now been commissioned to find out what was done well and what lessons need to be learned to improve patient safety.”
A study available on the medRxiv preprint server has estimated the increases of the SARS-CoV-2 Iota (B.1.526) variant for transmissibility, immune escape ability, and infection fatality rate in New York.
The study findings revealed that the Iota variant has considerably higher transmissibility and immune escape potential than previously circulating variants and that it can increase the infection fatality rate by 62%-82% among older adults.
Iota variant emerges in New York The lota variant, was first identified in New York City in November 2020 and then spread across the US and to 27 countries.
According to one lab study, the Iota variant modestly resists neutralisation by therapeutic monoclonal antibodies and vaccine/infection-induced antibodies. Fortunately however, evidence indicates that the variant does not increase the risk of breakthrough infections in vaccinated or previously infected individuals.
Study design For the study, the scientists analysed multiple epidemiological and population datasets collected in New York City and, with mathematical modeling, estimated the transmission rate, immune evasion ability, and infection fatality risk of the Iota variant.
The prevalence of SARS-CoV-2 was estimated at 16.6% at the end of the first wave, and 41.7% at the end of the second wave. Infections in all age groups were seen with the second wave, compared to mostly older age groups in the first.
Transmissibility and breakthrough A rapid increase in Iota-infected cases was observed during the second pandemic wave. Before the variant was first discovered in a neighbourhood in early November 2020, a rise in cases was seen in that same neighborhood, which remained higher than other neighbourhoods until Iota became prevalent and raised the baseline.
The researchers estimated that Iota is 15–25% more transmissible than previously circulating variants, with breakthrough infections in 0–10% of the population. These increases made the Iota variant become dominant in New York City from November 2020 to March 2021. Afterward, with the surge of more infectious variant B.1.1.7 (Alpha), Iota prevalence fell.
Despite a reduction in mortality rate following mass vaccination, infection fatality rate was seen to increase in New York City during the second pandemic wave. The researchers estimated that the Iota variant increases the infection fatality rates by 46% for 45–64 year olds, 82% (65–74), and 62% (75+). Compared to previously circulating variants, the Iota variant, overall, caused a 60% increase in infection fatality rate, comparable to that estimated for the Alpha variant.
The South African Human Rights Commission (SAHRC) said that it had been inundated with complaints from people who claimed they had been pressured to get a COVID vaccination. These included tenants being threatened with eviction if they did not vaccinate.
Employees and tenants were not the only ones coming forward; companies had also approached the commission for guidance and clarification. The situation is something of a grey area for employers, according to the Department of Labour’s director-general Thobile Lamati.
“Can the employer then force the employees to take the vaccine? This is a very difficult question because we have different work places and different situations,” he said, adding that a worker could not be compelled to take a vaccine without the risks being explained to them.
On Friday morning, acting Health Minister Mmamaloko Kubayi-Ngubane reiterated that vaccination was purely voluntary and confirmed that employers were not allowed to force workers to get the jab.
Nevertheless, the government is encouraging all eligible South Africans to achieve herd immunity, and exit the cycle of waves and economically crippling lockdowns. However, inoculation has not been mandatory, with a target of around two-thirds of the population vaccinated. In a statement, President Cyril Ramaphosa said that only one in ten South Africans now believed that COVID vaccines were not safe.
The commission’s Buang Jones said they would also be examining the reasons given by some people refusing to receive vaccines.
“The reasons may range from medical, religious or other constitutional accounts. But complainants will be requested to take us through their reasoning and the team will be offering advice to those who have complained to the commission.”
Jones said that companies also voiced concerns about the negative impact that COVID had had on their finances.
“That they would like staff to be at work and ensure that the company functions optimally. Their concern is there will be increases in sick leave taken and it will affect production. They are also concerned about the rights of other employees to have chosen to take the vaccine.”
Three-quarters of COVID cases were among the vaccinated in an outbreak during large public gatherings at Cape Cod island in Massachusetts, USA, with the Delta variant predominating, researchers found. This helped reverse the official US recommendations on mask-wearing when fully vaccinated.
Of 469 cases linked to numerous summer events and large summer gatherings in a small town, 346 (74%) happened in fully vaccinated people, and almost 80% of those cases were symptomatic, reported Catherine Brown, DVM, of the Massachusetts Department of Public Health, and colleagues.
There were five hospitalisations, four among fully vaccinated people, and no deaths. Of 133 cases with sequence information available, 89% were from the Delta variant, the authors wrote in the Morbidity and Mortality Weekly Report.
Moreover, vaccination coverage in Massachusetts was reported to be 69% as of July 3, they noted.
The data on this outbreak, along with the Delta variant, reportedly motivated the Centers for Disease Contol’s change in indoor masking guidance earlier last week.
CDC Director Rochelle Walensky, MD, noted that “rapid receipt and review of unpublished data” contributed to the guideline change.
The authors found that RT-PCR cycle threshold (Ct) values in vaccinated and unvaccinated people were comparable. Pointing to these high viral loads among the vaccinated in this case, Dr Walensky said it suggested that vaccinated individuals infected with Delta could still transmit the virus.
“This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation,” she said, saying it was “updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones.”
The authors detailed increased reports of COVID cases in Barnstable County, beginning on July 10, including those who were fully vaccinated. Many COVID patients reported attending large indoor and outdoor gatherings.
They identified a cluster of cases, 346 of them fully vaccinated, in Massachusetts residents, who tested COVID positive 14 days or sooner after travel to or staying in Barnstable County.
Cases were also reported from residents of other states who traveled to this town during the period, and reports of secondary transmission.
The authors urged stricter prevention measures at such events, given, “the potential risk of infection during attendance at large public gatherings that include travelers from many areas with differing levels of transmission.”
Journal information: Brown CM, et al “Outbreak of SARS-CoV-2 Infections, Including COVID-19 Vaccine Breakthrough Infections, Associated with Large Public Gatherings – Barnstable County, Massachusetts, July 2021” MMWR 2021; Published July 30, 2021.
A study of COVID transmission in a US university indicates that at least a fifth of asymptomatic COVID cases passed the infection on, suggesting that containing the SARS-CoV-2 virus is more difficult than previously thought.
Even as multiple resurgences of the COVID pandemic occur in many countries, the risk posed by various degrees of infection, from asymptomatic through presymptomatic to symptomatic SARS-CoV-2 infection is still undefined.
Researchers in a new study found that at least a third of infected individuals were asymptomatic, and a fifth of them successfully passed on the virus to others. In comparison, about a quarter of symptomatic cases did so, especially when the symptoms included fever, cough and shortness of breath. In these instances, up to 30% of them transmitted the virus. The findings of the study are available on the medRxiv preprint server, and are not yet peer reviewed.
Asymptomatic carriage has been suspected and reported since the pandemic’s outset, though the exact proportion has been described to be anywhere between 40% and 77%. Such differences in ascertainment could be from varying demographic, testing, and interaction patterns.
Most systematic reviews have quoted figures of 30-45% as asymptomatic, which are considered to be much less transmissible. Secondary attack rates (SAR) of as low as 0.7% are reported in these cases, compared to 21% for symptomatic cases. Most of these studies had a lack of follow-up, meaningthat asymptomatic and presymptomatic cases could have been combined or misclassified.
The study was based on a campus population at a Midwestern university in the USA, during the northern hemisphere autumn of 2020. The study included about 13 000 students and 1600 confirmed cases.
The researchers found not only that SAR differed when patients were classified by symptom presence or absence, but the nature of the symptoms also determined the SAR. This could hamper containment efforts, especially as economic activity is burgeoning after a long hiatus.
Moreover, daily checks on individuals such as employees and students which are meant to help define transmission risk, may not yield the desired results when the wide difference in symptom types and timing in relation to infection, as well as the SAR, are taken into account.
The college campus in this study had a set of measures in place to reduce the risk of viral spread. In particular, this included a daily requirement to assess one’s own health and report on any issues; testing for the virus; contact tracing; case isolation and contact quarantine; and surveillance testing to monitor the spread of the virus.
In August, students began to come back to campus, and this was associated with 151 positive tests for the virus by reverse transcriptase polymerase chain reaction (RT PCR). This led to the suspension of in-person classes, with all teaching being online until August 24.
At this point, graduate and professional students again had in-person classes, while the others continued with online classes until September 2. The number of cases detected by a positive PCR test went down, from just over 600 in August to approximately 150 by September.
A second rise was seen in the middle of October, and by the end of the semester, on November 20, over 1500 students were positive. Of these, daily health data was present for at least half the days for about 1200 students.
Testing was administered by a rapid antigen test if a student had symptoms suggestive of SARS-CoV-2 infection or had a history of exposure to someone with COVID. Negative results were validated by a PCR, with the result coming within 1-2 days during which time they were quarantined.
Positive rapid antigen tests led to 14 days isolation, along with contact tracing and quarantine. The quarantined students were also advised to do an antigen test, which, if positive, necessitated a PCR test. They were also expected to quarantine until tests on days four and seven were also negative, at which point they were released. The university also conducted surveillance monitoring to hopefully catch asymptomatic or presymptomatic cases.
The scientists found that nasal congestion, headache and dry cough was higher among those who tested positive within five days, with fever and sensory disturbances (anosmia/dysgeusia/ageusia) were higher among positives only three and two days previous to testing positive. Loss of taste and smell occurred in the greatest proportion of cases by day four after testing positive.
The SAR was 19% vs 25% for asymptomatic vs symptomatic index cases on day 14 after virus exposure. In the four days immediately following exposure, symptomatic cases showed a higher SAR, with presymptomatic cases showing lower SAR and asymptomatic cases the lowest.
Perhaps this was because symptomatic cases have delayed testing, indicating that they had more time to transmit the virus before testing positive.
After this period, presymptomatic COVID had the highest SAR, but symptomatic case SAR began to rise, becoming almost identical thereafter. At seven days post-exposure, the SAR in all categories flattened, therefore making it the quarantine limit for contacts.
Symptoms varied in their predictive capacity, but fever, shortness of breath and a dry cough at the onset of infection were found to be associated with an SAR of 30%, provided body aches and/or chills were also reported. The investigators suggest this could be owing to higher viral loads, and therefore greater ability to shed and spread the virus.
The authors concluded that daily virus surveillance does not by itself help in containing transmission. The only way out may be rigorous face mask use until sufficient vaccine coverage is reached, along with social distancing, testing and quarantine. Both asymptomatic and presymptomatic viral transmission are significant contributors to viral spread, impeding efforts to stop the virus, especially with novel emerging SARS-CoV-2 strains.
Upgrading face masks to filtering face piece (FFP3) respirators for healthcare workers on COVID wards produced a dramatic reduction in hospital acquired SARS-CoV-2 infections, according to a preliminary study published in the BMJ.
For most of 2020, Cambridge University Hospitals NHS Foundation Trust followed national guidance that healthcare workers should use fluid resistant surgical masks as respiratory protective equipment unless aerosol generating procedures (AGPs) were being carried out when FFP3 respirators were advised.
From the pandemic’s outset, the trust has been regularly screening its healthcare workers for SARS-CoV-2 even when asymptomatic. They found that healthcare workers on “red” COVID wards had a greater infection risk than staff on “green” wards, even with protective equipment. So in December 2020 the trust implemented a change in policy so that staff on red wards wore FFP3 masks instead of fluid resistant surgical masks. The FFP3 standard requires that masks filter 99% of all particles measuring up to 0.6 μm.
The study was carried out at Addenbrooke’s Hospital in Cambridge. Before the change in policy, cases among staff were higher on COVID versus non-COVID wards in seven out of eight weeks analysed. Following the change in protective equipment the incidence of infection on the two types of ward was similar. Of 609 positive results over the eight week study period, 169 were included in the study. Healthcare workers who were not ward based or worked between different wards were excluded, as were, non-clinical staff, and staff working in critical care areas.
The researchers developed a simple mathematical model to quantify the risk of infection for healthcare workers. This found that the risk of direct infection from working on a red ward prior to the policy change was 47 times greater than the corresponding risk from working on a green ward. While almost all cases on green wards were likely caused by community-acquired infection, cases on red wards at the beginning the study period were attributed mainly to direct, ward-based exposure.
The model also suggested that the introduction of FFP3 respirators provided 100% protection (confidence interval 31.3%, 100%) protection against direct, ward based covid infection.
Study author Chris Illingworth, from the MRC Biostatistics Unit at the University of Cambridge, said: “Before the face masks were upgraded, the majority of infections among healthcare workers on the COVID wards were likely because of direct exposure to patients with COVID. Once FFP3 respirators were introduced, the number of cases attributed to exposure on COVID wards dropped dramatically—in fact, our model suggests that FFP3 respirators may have cut ward based infection to zero.”
Michael Weekes from the department of medicine at the University of Cambridge added: “Our data suggest there’s an urgent need to look at the PPE offered to healthcare workers on the frontline. Upgrading the equipment so that FFP3 masks are offered to all healthcare workers caring for patients with COVID could reduce the number of infections, keep more hospital staff safe, and remove some of the burden on already stretched healthcare services caused by absence of key staff because of illness.”
Colorized scanning electron micrograph of an apoptotic cell (purple) heavily infected with SARS-COV-2 virus particles (yellow), isolated from a patient sample. Image captured at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. Credit: NIAID
A preliminary study has possibly determined why the SARS-CoV-2 Delta variant is more infectious and pathogenic than its ancestor.
Through a series of in vitro experiments, researchers have discovered that variant’s enhanced ability to induce cell-to-cell fusion (syncytia) and reduced susceptibility to vaccine and infection-induced antibodies together help make the Delta variant more infectious than previously circulating variants. The study, which is yet to be peer reviewed, is currently available on the bioRxiv preprint server.
The SARS-CoV-2 virus has undergone more than 12 000 mutations since it was first detected in December 2019, most of which are neutral and do not contribute to viral evolution. However, the acquisition of specific mutations in structural and non-structural proteins has caused the emergence of novel, more virulent SARS-CoV-2 variants.
Spike protein mutations are particularly concerning as they can significantly influence viral infectivity, virulence, and immune evasion ability.
The B.1.617 lineage drove a massive surge in new COVID cases in India. This lineage is further divided into three sub-lineages, namely B.1.617.1, B.1.617.2, and B.1.617.3. Although these emerged first in India, the B.1.617.2 or Delta variant or soon became dominant in many countries, including South Africa where it has driven a new surge of infections, particularly in Gauteng Province. The World Health Organization (WHO) has designated the Delta variant as a ‘Variant of Concern’ (VOC) due to its significantly increased infectivity and pathogenicity.
In the current study, the scientists have evaluated the susceptibility of the Delta variant to neutralisation by vaccine or natural infection-induced antibodies.
Delta variant mutations
The Delta variant’s spike protein contains nine mutations in the S1 subunit and one mutation in the S2 subunit. In the S1 subunit, five mutations are present in the N-terminal domain containing binding sites (epitopes) for neutralising antibodies. In addition, two mutations are present in the receptor-binding domain of the S1 subunit, which is known to influence antibody-mediated neutralisation and infectivity. Among the three remaining mutations, two are known to increase angiotensin-converting enzyme 2 (ACE2) binding, viral replication, and spike protein cleavage at the S1/S2 site.
Delta variant host cell entry
Using African green monkey and human cells, the researchers found that Delta can enter kidney cells of both species with similar efficacy as the wild-type SARS-CoV-2. However, for human colon and lung cells, Delta showed 1.5-fold and 2-fold higher invading ability, respectively, compared to the wild-type virus. Since the Delta variant spike protein did not exhibit increased ACE2 binding, the scientists suggest that increased entry of B.1.617.2 into colon and lung cells is not mediated by enhanced ACE2 binding.
Besides inducing fusion between the viral envelope and host cell membrane, the spike protein triggers the fusion of infected cells with nearby cells to form large multinucleated cells, known as syncytia. Given the fact that spike-induced syncytia formation contributes to COVID pathogenesis, the scientists investigated whether Delta variant infection is associated with increased syncytia formation.
By conducting in vitro experiments on human lung cells expressing high levels of ACE2, they found that Delta spike expression leads to 2.5-fold higher and larger syncytia formation than the wild-type spike expression.
Delta variant’s immune evasion ability less than Beta?
The scientists tested the ability of four therapeutic monoclonal antibodies to neutralise the Delta variant, of which only Bamlanivimab failed. The other three antibodies exhibited similar efficacy in neutralising both wild-type virus and Delta variant.
Antibodies derived from COVID recovered patients, and BNT162b2-vaccinated individuals showed only slightly reduced efficacy in neutralising the Delta variant as compared to the wild-type virus. In contrast, the B.1.315 or Beta variant, first detected in South Africa, showed a significantly higher ability to evade infection- and vaccination-induced immunity.
In summary
The study showed that Delta’s increased ability to invade lung cells may enhance infectivity and pathogenicity. Though it has lower susceptibility to antibody-mediated neutralisation, it is possible that Delta may be effectively controlled by immunity developed in response to natural infection or vaccination.
Journal information: Arora P. 2021. Increased lung cell entry of B.1.617.2 and evasion of antibodies induced by infection and BNT162b2 vaccination. bioRxiv. https://www.biorxiv.org/content/10.1101/2021.06.23.449568v1
There might be an unexpected benefit to the rhinovirus, or the most frequent cause of the common cold — protection against COVID, according to a study at Yale University.
Around 200 viruses cause the common cold, of which rhinovirus is the most common. Researchers found that the rhinovirus kick-starts interferon-stimulated gene activity. Within airway tissues infected with the rhinovirus, this also can halt replication of the SARS-CoV-2 virus.
Setting off these defences early in the course of COVID infection might prevent or treat the infection, said Ellen Foxman, assistant professor of laboratory medicine and immunobiology at the Yale School of Medicine and senior author of the study. One method is treating patients with interferons, an immune system protein which is also available as a drug.
“But it all depends upon the timing,” Prof Foxman clarified.
In later stages of COVID, high interferon levels correlate with worse disease and may fuel overactive immune responses, according to previous research. But recent genetic studies show that interferon-stimulated genes may actually also be protective in cases of COVID infection.
Prof Foxman’s lab wanted to study this defence system early in the course of COVID infection.
Earlier studies by the lab had shown that common cold viruses may protect against influenza, so they decided to find out whether rhinoviruses would have the same beneficial impact against the COVID virus. The researchers infected lab-grown human airway tissue with SARS-CoV-2 and found that for the first three days, viral load in the tissue doubled about every six hours. However, replication of the coronavirus was completely halted in tissue which had been exposed to rhinovirus. When antiviral defences were blocked, the SARS-CoV-2 could replicate in airway tissue previously exposed to rhinovirus.
The same defences slowed down SARS-CoV-2 infection even without rhinovirus, but only with a low infectious dose, suggesting that the viral load at the time of exposure affects whether the body can effectively fight the infection.
The researchers also studied nasal swab samples from patients diagnosed close to the start of infection. They found evidence of rapid growth of SARS-CoV-2 in the first few days of infection, followed by activation of the body’s defenses. According to their findings, the virus typically increased rapidly for the first few days of infection, before host defenses kicked in, doubling about every six hours; in some patients the virus grew even faster.
“There appears to be a viral sweet spot at the beginning of COVID, during which the virus replicates exponentially before it triggers a strong defence response,” Foxman said.
Interferon treatment is promising but could be tricky, she said, because it would be mostly effective in the days immediately after infection, when many people are asymptomatic. In theory, interferon treatment could be used prophylactically in people at high risk who have been in close contact with others diagnosed with COVID. Interferon is being trialled in COVID, and there appears to be a benefit when given early, but not late.
The study helps explain why influenza infections are lowered at times of the year when the common cold is prevalent, Prof Foxman said. The easing of social distancing measures could cause the common cold and flu viruses, which have been suppressed, to spring back with greater force. Respiratory viruses interference with each other could be a mitigating factor, creating an ‘upper limit’ on the degree to which respiratory viruses circulate together, she said.
“There are hidden interactions between viruses that we don’t quite understand, and these findings are a piece of the puzzle we are just now looking at,” Prof Foxman said.
Journal information: Cheemarla, N.R., et al. (2021) Dynamic innate immune response determines susceptibility to SARS-CoV-2 infection and early replication kinetics. Journal of Experimental Medicine.doi.org/10.1084/jem.20210583.
As COVID cases and deaths continue to rise in the Free State, with schools being closed, it is unclear when the province’s teachers will receive their vaccinations.
The deaths of six learners, 75 teachers, and three support staff from COVID have been reported in the Free State since March 2020.
While teachers await their vaccines, COVID still claims lives in the school system – and not just older teachers and staff. Quincy Tsoenyane lost a daughter to COVID-related complications, 18 year-old Nomthandazo Ngcoyi, who was a learner at Lephola Secondary school in Welkom. Nomthandazo was one of 11 learners at the school who tested positive for COVID in May. Tsoenyane, who is a father to two surviving children, said it pains him to know that his daughter got sick at school.
According to the Department of Basic Education (DBE), Nomthandazo developed a cough at school and was tested for COVID along with other learners. On 19 May, she tested positive and was sent home to self-isolate. She died at home six days later.
A rare case
Dr Cloete van Vuuren, an Infectious Disease Specialist in the Department of Internal Medicine at the University of Free State, said that Nomthandazo’s death is a rare case as it is uncommon for young people to die from the SARS-CoV-2 virus.
The DBE figures show that since March 2020, the Free State has recorded a total of 2101 positive cases among teachers in schools: 1377 among learners, and 461 among non-teaching staff. Outbreaks of COVID cases have forced several shutdowns of Free State schools.
Holding out for vaccines
As COVID numbers climb in Free State schools, teaching federations and unions are urging that teachers be vaccinated as soon as possible.
From 26 July, children from Grades R to 7 will return to in-person classes. In a media statement, the National Professional Teachers Organisation of South Africa (Naptosa) said that they are pleased to hear that the education sector will receive 500 000 doses of the Johnson & Johnson vaccine.
However, the union said they are still in limbo because the doses must still require verification by the Food and Drug Administration (FDA) and will expire on 28 July.
As of Thursday, there were 591 new COVID cases in Free State, with a new case incidence rate of 17.8 per 100 000 people.
Teachers need to protect themselves and others
Dr Kerrin Begg, Public Health Specialist in the Faculty of Health Sciences at the University of Cape Town reminded teachers that although it is understandable for them to be anxious about the vaccination, each and every person has the responsibility to educate themselves.
“Teachers need to be teaching themselves about the virus just like they do in their everyday line of work of teaching children.
“At the Colleges of Medicine of South Africa, we have produced school guidelines on measures to take to reduce the transmission of COVID in the school environment,” she said. She said that socialising outside of class was where most of the transmission took place, and that learners now no longer adhered to social distancing.
“We remind parents and teachers to remember that protecting themselves is not to be practiced during school hours only, but there are three major focal points of transmission which are before, during, and after school hours.
“Teachers need to understand that the environment of the classroom is very important. Fresh air is better than artificial air, outside is better than inside. Schools also need to continue to promote personal and physical distancing, and hygiene measures daily,” Dr Begg said.
Researchers have demonstrated that normal breathing can transport viruses in saliva droplets up a distance of up to 2.2 metres in 90 seconds.
The World Health Organization and the Centers for Disease Controlrecommend social distancing to prevent the spread of COVID. The distances are estimated from various studies, but there is a need for further research into how viruses are transported from one person to another. Previous studies considered aerosol transport after coughing or sneezing, while this study focused on normal human breathing, using computer simulations with a more realistic model than prior studies. A normal breath produces periodic jet flows that contain saliva droplets, but those jets’ velocity is less than a tenth that of a cough or sneeze.
Wearing a face mask greatly reduces the distance which these droplets can travel. Saliva droplets restricted by a mask had travelled only 0.72 metres after two minutes, far short of the distance of 1.8 metres suggested by the CDC.
The investigators found even normal breathing produces a complex field of vortices that can move saliva droplets away from the person’s mouth. The role of these vortices has not previously been understood.
Study author, Ali Khosronejad, American Institute of Physics said: “Our results show that normal breathing without a facial mask generates periodic trailing jets and leading circular vortex rings that propagate forward and interact with the vortical flow structures produced in prior breathing cycles.”
This complex vorticity field can enable the transport of aerosol droplets over long distances despite the slow speeds. A face mask serves to dissipate the kinetic energy of the jet produced by an exhaled breath, thereby disrupting the vortices and limiting the travel of virus-laden droplets.
The researchers also took into account evaporation of the saliva droplets. With no mask, they found the saliva droplets near the front of the plume of exhaled breath had partially evaporated, reaching a size of only one-tenth of a micrometre. In stagnant indoor air, it would take days for droplets this small to settle to the ground.
Masks partially redirect the exhaled breath downward, significantly restricting forward motion of the plume, so the risk of suspended droplets remaining in the air is substantially reduced.
“To simplify the breathing process, we did not consider the flow of air-saliva mixture through the nose and solely accounted for the flow through the mouth,” Khosronejad said. “In future studies, we will explore the effect of normal breathing via both the nose and mouth.”
Journal reference: Khosronejad, A., et al. (2021) A computational study of expiratory particle transport and vortex dynamics during breathing with and without face masks. Physics of Fluids. doi.org/10.1063/5.0054204.
