Again: unpublished data, no details, no peer review, science-by-press-release. That ain’t good. But big, as political writers sometimes say, if true. People infected with the virus but without symptoms—asymptomatic spreaders—seem to be a reason the disease is pandemic-y. Nobody’s sure how big a reason, though.

Lots of other respiratory viruses overlap symptoms and transmission—sometimes the symptoms themselves, like coughing, are the way the virus gets from an infected person to others. The time between infection and symptoms, called the incubation period, doesn’t last long. “We know with flu, the incubation period is relatively short, and people may shed virus for a day or so,” says Arnold Monto, an epidemiologist at the University of Michigan who chairs the FDA’s Vaccines and Related Biological Products Advisory Committee, which helps make decisions on approving new vaccines. “We can infect a ferret with flu and they get sick, but if they’re not coughing or doing whatever ferrets do when they’re symptomatic, they don’t transmit as well.”

The assumption that this was also true for Covid-19 provided the stitching for a lot of pandemic protection cosplay—like temperature checks and symptom surveys. “A lot of the things we did early were based on the fact that with traditional SARS, there was not a whole lot of transmission from asymptomatic individuals,” Monto says. “Symptomatic people tend to transmit more than asymptomatic people for respiratory infections. We think that’s probably true with Covid, but it is becoming more clear that asymptomatic people are also involved in transmission.”

The problem is, a Covid-19 vaccine that only prevents illness—which is to say, symptoms—might not prevent infection with the virus or transmission of it to other people. Worst case, a vaccinated person could still be an asymptomatic carrier. That could be bad. More younger people tend to get the virus, but more older people tend to die from it; socioeconomic status and ethnicity also have an impact on death rates. Some people have relatively light symptoms; other people have symptoms that hang on for months. And perhaps most importantly, a vaccine is the only way to reach herd immunity without a bloodbath. As politicized as the notion has become, herd immunity is essentially the sum of direct protection—what you might get if you’re vaccinated—and indirect protection, safety afforded by the fact that people around you aren’t transmitting the disease to you because they either already had the disease themselves or because they got vaccinated against it. If vaccinated people can still be asymptomatic spreaders, that means less indirect protection for the herd.

That really matters, because there isn’t enough vaccine to go around. Not yet, anyway. Some groups of people will go first. The characteristics of the available vaccines would, in a perfect world, determine who those people should be. One that only prevented illness might go first to the elderly, in whom severe illness is more likely to lead to death. One that prevented infection and transmission might go to essential workers and frontline caregivers. “Part of our worry is, we want to get it right in the early allocation phase, making sure we’re targeting the vaccine as best as you can,” says Grace Lee, a professor of pediatrics at Stanford School of Medicine and member of the CDC’s Advisory Committee on Immunization Practices. “If the only thing it did was protect against severe disease, you’d want to look at the population that has severe disease and only use it there, and nowhere else.”

That’s almost certainly not going to be the situation. The vaccines will probably all have some effect on transmission. But right now no one knows how much, or which one is better, or for whom—because so far only AstraZeneca has even a hint of data studying the problem.


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