Is the coronavirus improving in airborne transmission?

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Newer variants of the coronavirus such as alpha and delta are highly contagious, infecting many more people than the original virus. Two new studies offer a possible explanation: the virus evolves to spread more efficiently in the air.

The realization that the coronavirus is airborne indoors has transformed efforts to contain the pandemic last year, sparking heated debates over masks, social distancing and ventilation in public spaces.

Most researchers now agree that the coronavirus is mainly transmitted by large droplets that fall quickly to the ground and much smaller droplets, called aerosols, which can float longer distances indoors and settle directly in. the lungs, where the virus is most harmful.

The new studies do not fundamentally change this point of view. But the results signal the need for better masks in certain situations and indicate that the virus is evolving in a way that makes it more formidable.

“This is not an Armageddon scenario,” said Vincent Munster, a virus expert at the National Institute of Allergy and Infectious Diseases, who led one of the new studies. “It’s like modifying the virus for more efficient transmission, which I think we all sort of expected, and now we are seeing it happening in real time.”

Munster’s team showed that small aerosols traveled much longer distances than larger droplets and that the alpha variant was much more likely to cause new infections through aerosol transmission. The second study found that people infected with alpha exhaled about 43 times more virus in tiny aerosols than those infected with older variants.

The studies compared the alpha variant with the original virus or other older variants. But the results may also explain why the delta variant is so contagious – and why it displaced all other versions of the virus.

“This really indicates that the virus is evolving to become more efficient at transmission by air,” said Linsey Marr, an airborne virus expert at Virginia Tech who was not involved in either of the two studies. “I wouldn’t be surprised if, with delta, this factor were even higher. “

The ultra-transmissibility of variants can be summed up as a mixture of factors. Lower doses of the variants may be needed for infection, or the variants may replicate faster, or more of the variant virus may be exhaled in aerosols – or all three.

The alpha variant has been shown to be twice as transmissible as the original virus, and the delta variant has mutations that further enhanced its contagiousness. As the virus continues to change, the new variants may prove to be even more transmissible, experts said.

But the tools at our disposal still work well to stop the spread. Even loose clothing and surgical masks block about half of fine aerosols containing the virus, according to the study of people infected with variants, published this month in the journal Clinical Infectious Diseases.

Still, at least in some crowded spaces, people may consider switching to more protective masks, said Don Milton, an aerosol expert at the University of Maryland who led the research.

“Since it appears to be moving towards a better generation of aerosols, then we need better containment and better personal protection,” Milton said of the virus. “We recommend that people switch to more fitted masks. “

To compare how different variants spread through the air, her team asked participants with mild or asymptomatic infections to recite the alphabet, sing “Happy Birthday” out loud, or shout the slogan of the University of Maryland, “Go, Terps!”

People infected with the alpha variant had large amounts of the virus in their nose and throat, much more than those infected with the original virus. But even after adjusting for this difference, people infected with the variant released about 18 times as much virus in the smaller aerosols.

But the researchers only looked at four people infected with alpha and 45 with older variants. This could skew the differences seen between the variants, said Seema Lakdawala, a respiratory virus expert at the University of Pittsburgh, who was not involved in any of the new studies.

People who are infected can pass the virus to many, many, or even none. The amount of virus they expel can depend on where it replicates in the airways, the nature of the mucus in its environment, and other microbes it can hitchhike with.

“We really have no idea why some people are super-spreaders and others are not,” Lakdawala said. “There is a lot of heterogeneity between individuals.

Data from a larger number of participants would be more compelling, but the two studies together suggest that aerosol-enhanced transport contributes at least in part to the contagiousness of the variant, she said.

The Munster study did not involve people at all, but Syrian hamsters. Using the animals allowed the team to tightly control the experimental conditions and focus only on the movement of aerosols, Munster said.

The researchers separated pairs of hamsters with tubes of varying lengths that allowed air circulation but no physical contact. They examined the extent to which the different variants traveled from infected “donor” hamsters to uninfected “sentinel” hamsters.

When the cages were more than 2 meters apart, only the smallest aerosols – particles smaller than 5 microns – were found to infect the sentinel hamsters. And the team discovered, as expected, that the alpha variant outperformed the original virus by infecting sentinel hamsters.

The results were published on bioRxiv, a website that features articles before they are published in a scientific journal.

Researchers are now testing the delta variant and expect to find that it is even more effective, Munster said.

Together, the new findings underscore the importance of masks for those vaccinated, especially in crowded spaces, experts said. Although people with chronic infections after vaccination are much less likely to spread the virus than unvaccinated people, the contagiousness of the variants increases the likelihood.

With billions of people vaccinated around the world and billions still unvaccinated, the virus can still change in unexpected ways, Munster said: “There could be additional evolutionary pressures, shaping the evolutionary direction of this virus. “

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