Is SARS-CoV-2 soon to become airborne?

SARS-CoV-2 has evolved in a way that makes the virus more capable at spreading, shunning antibodies, or both. What is more is that this change appears to be favouring a more efficient viral aerosol generation, as shown in a recent study.

Specifically, patients infected with the Alpha variant exhaled infectious viral aerosols, putting 18 to 100 times more virus into the air compared with those who had the original strains of the virus. The study was conducted from May 2020 through April 2021, during which the Alpha variant was the dominant strain that circulated. [Clin Infect Dis 2021;doi:10.1093/cid/ciab797]

“[The finding] provides further evidence of the importance of airborne transmission,” said one of the study authors Dr Donald Milton, professor of environmental health at the University of Maryland School of Public Health, Maryland, US.

“We know that the Delta variant circulating now is even more contagious than the Alpha variant. Our research indicates that the variants just keep getting better at travelling through the air, so we must provide better ventilation and wear tight-fitting masks, in addition to vaccination, to help stop spread of the virus,” Milton added.

In the study, Milton and colleagues enrolled 61 patients with active SARS-CoV-2 infection, among whom 57 had known serologic status (49 seronegative, 8 seropositive; mean age 23.8 years, 35 percent female), from 0 to 12 days after the onset of symptoms or first positive test. All patients, who were asymptomatic or mild at the time of study, gave blood, saliva, mid-turbinate (each nostril) and fomite (mobile phone) swabs, and 30-minute breath samples while vocalizing into a Gesundheit-II (taken with and without masks) at up to two visits 2 days apart.

Among seronegative patients, four (8 percent) were infected with the Alpha variant; none had the Delta or other variants associated with increased transmissibility. Each of the Alpha variant-infected patients had detectable concentrations of viral RNA in all mid-tubinate, saliva, fomite, and aerosol samples.

Looking at the aerosol samples from the four Alpha-variant cases, SARS-CoV-2 RNA was present in 45 percent of fine (≤5 µm), 31 percent of coarse (>5 µm) aerosols, and 65 percent of fomite samples. Bivariate analyses of samples collected without masks showed that the Alpha variant was associated with a 43-fold (95 percent confidence interval [CI], 6.6–280) increase in fine aerosol viral RNA as compared with earlier strains and variants, and this fine-aerosol shedding remained significantly greater (18-fold, 95 percent CI, 3.4–92) despite adjusting for viral RNA in saliva, swabs, and other potential confounders. Two fine aerosol samples, collected while participants wore masks, were culture-positive.

When the impact of Alpha variants on shedding was analysed in the larger data set, including samples collected with and without masks, the variant correlated with an increase of 100-fold (95 percent CI, 16–650) in coarse-aerosol and 73-fold (95 percent CI, 15–350) in fine-aerosol RNA shedding.

“[It is established already] that virus in saliva and nasal swabs was increased in Alpha-variant infections. Virus from the nose and mouth might be transmitted by sprays of large droplets up close to an infected person. But our study shows that the virus in exhaled aerosols is increasing even more,” noted study co-author Jianyu Lai, a doctoral student at University of Maryland School of Public Health.

In the East as in the West

The viral RNA copy numbers and overall rates of positive breath samples observed during singing and loud talking observed in the present study were consistent with those obtained from viral RNA aerosol samples collected from 22 COVID-19 patients (including those with the Alpha, Beta, Kappa, and Delta variants) in Singapore (51 percent vs 59 percent, respectively, during singing and loud talking). However, majority (68 percent) of the patients studied in Singapore were infected by variants associated with increased transmissibility, and each patient was sampled on only 1 day. [medRxiv 2021;doi:10.1101/2021.07.15.21260561]

“[The Asian cohort study’s] and our data are in agreement that among persons infected with ‘wild-type’ strains, a minority (26 percent and 31 percent, respectively) shed detectable levels of viral RNA into aerosols. However, our data on Alpha-variant infections and the data from Singapore suggest that this is changing, and most cases may now be shedding viral aerosols more frequently,” according to the authors of the US study.

Nevertheless, the present study does not paint a completely grim picture. The authors found that countermeasures—loose-fitting cloth and surgical masks, in particular—cut down the amount of virus that got into the air around infected patients by about half. 

Masks reduced viral RNA by 48 percent (95 percent CI, 3–72) in fine and by 77 percent (95 percent CI, 51–89) in coarse aerosols, with the protection conferred by cloth and surgical masks not significantly different.

“The take-home messages from [our] paper are that the coronavirus can be in your exhaled breath, is getting better at being in your exhaled breath, and using a mask reduces the chance of you breathing it on others,” said Dr Jennifer German, study co-author.

The virus’ evolution has sprung up variants that demand to be taken seriously and have accelerated the COVID-19 pandemic despite the development and subsequent rollout of vaccines. This means that a layered approach is necessary to control the spread of the infection (ie, improved ventilation, filtration, air sanitation, and use of high-quality tight-fitting face masks or respirators), as the authors pointed out.

“This will be especially true where vaccination rates are low, vaccine is not available, and for people with poor immune responses or waning immunity… [O]ur data support community mask mandates and tight-fitting masks or respirators for workers in healthcare but also in all workplaces where people are sharing indoor air or have frequent public contact.”