In fact, according to basic outbreak math, if the vaccine stops anything less than two-thirds of transmission events, it’s impossible to reach herd immunity at all. That threshold is commonly thought to be about 70% of the population.īut if vaccinated people are “leaky”-if they can still spread the virus sometimes-the threshold will rise. One way the pandemic can end is via “herd immunity”-that is, when enough people are vaccinated, or infected, for the outbreak to recede on its own because there aren’t enough people left to infect. Researchers know that stopping transmission is the only way to get rid of the coronavirus for good. “Because we may need to turn our attention to the kinds of vaccines that do reduce transmission.” Stemming the flood He believes the study is worth the effort. Corey says the group has since updated the proposal and that it again being considered by the National Institutes of Health. On December 31, the Wall Street Journal reported that the proposed study had failed to win funding, because of high costs and questions over its feasibility. “Then close contact tracing could estimate how often the people spread the virus, which is known as forward transmission.” “You can learn a lot by understanding the acquisition and viral titers in the nose,” says Corey. Then, with contact tracing, they hoped to map how often vaccinated students spread the virus. They proposed “almost daily” nose swabs to monitor exactly when the virus appeared, and in what amounts, in the airways of both vaccinated and unvaccinated students. To figure that out, researchers at the Covid-19 Prevention Network last year proposed studying more than 20,000 students on two dozen US campuses, including Louisiana State University. While looking for the virus in people’s noses can detect silent infections, it doesn’t actually prove whether these people can then infect others. Moderna suggested that “that some asymptomatic infections start to be prevented after the first dose.” However, preliminary data the company submitted to the US Food and Drug Administration in December offered one clue: people who got one dose of the vaccine were 66% less likely to turn up positive on a coronavirus test than those who got the placebo. Moderna Therapeutics, maker of another vaccine, did not reply to questions about whether it is studying transmission. In a report published January 7, a team including epidemiologists at the Centers for Disease Control and Prevention estimated that a third of people infected with the coronavirus never develop symptoms and that they cause about a quarter of all spread. The evidence now suggests that the role of such “silent spreaders” is substantial, even though they tend to infect fewer people on average. Early in the pandemic, researchers discovered that some people who caught the coronavirus and never felt sick were still spreading the disease. If you can cut down symptoms, you are probably cutting down transmission,” says Shaman.īut that doesn’t mean there is no spread. “There is strong evidence that contagiousness is correlated with symptoms. Overall, they have much less virus in their airways. For example, vaccinated monkeys spritzed with the virus do get infected but don’t become particularly sick. The evidence so far suggests that vaccines should cut the chance of transmission, but may not eliminate it. One model, published in August by a team at Emory University, found that a vaccine that’s good at stopping spread, but not very good at stopping disease, would still lead to fewer deaths overall because it would slow the outbreak enough to reduce the total number of people who get infected.Ī step toward understanding how often vaccinated people spread the virus is what Pfizer is doing now: trying to figure out whether people like De Toma are getting infected without ever feeling sick. What they didn’t measure was the “indirect” effect of vaccines in preventing the further spread of the virus, even though some computer models have predicted that blocking transmission could save more lives.
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