When might we know whether coronavirus vaccines limit transmission?
January 11, 2021 12:34 PM   Subscribe

All the news articles about the SARS-CoV-2 vaccine say that we still need to wear masks, keep distance, etc., because we don't know yet whether being vaccinated stops transmission of the virus. Well, when are we likely to know the answer to this?
posted by medusa to Science & Nature (6 answers total) 4 users marked this as a favorite
I think the biggest indicator we are waiting for are if cases plummet once we hit a large % of immunizations.

But, in most interviews with virologists, most people say that technically it's DEFINITELY possible (been observed in the past using mucous), but the amount/chances of transmission may be MUCH lower when you are immunized.

This is a pretty good article on the topic: https://fortune.com/2020/12/22/covid-vaccine-infectious-face-masks-transmission/

I think it's easy to jump to conclusions with that information, but I think caution is appropriate. I expect many people will assume it's impossible to transfer once vaccinated, but I would encourage everyone reading this to consider still being very cautious around people that are unvaccinated and high risk, even if they have the vaccine.

Other commonly misunderstood things - there are a few weeks after vaccination before immunity happens, and, it's possible that your immune system COULD forget how to make the antibodies. (Note, this is different than the active antibodies not being in your blood anymore, which isn't necessary for immunity).

I hope this information is clear - love to answer any questions as I have been following the discussion very closely.
posted by bbqturtle at 12:50 PM on January 11 [3 favorites]

Another way we might learn about it is through challenge trials, which don't happen in the US but have been proposed for the UK and other countries as of October or so (current status unknown).

In those cases, it would be possible to PCR test every day following a known exposure, and gather the data that would provide a more definitive answer about viral counts and locations.

Right now, with community spread at high levels, it's going to be hard to either single out or rule out vaccinated spread as a cause, so the usual epidemiological methods aren't going to be as helpful.
posted by Dashy at 1:06 PM on January 11 [1 favorite]

Emily Oster touches on this in her newsletter today. More that the link, but here's a couple of paragraphs:
We do not yet know details about whether and how much COVID-19 vaccination limits your ability to spread the virus. To be clear, there is a lot of reason to think that it would limit your ability to spread at least to some extent. Notably, asymptomatic infections seem to spread less than symptomatic ones. Evidence suggests that asymptomatic people are perhaps 75 or 80% less likely to spread the virus than symptomatic people (see a summary in Nature here, and this meta-analysis on household transmission).

When will we know more about this? The COVID-19 vaccine trials have as their primary endpoint illness with COVID-19. But the trials are also collecting secondary endpoints which will help understand whether they block infection. This includes collecting swabs to look for evidence of virus in the upper respiratory tract, and also testing for evidence of antibodies to other part of the virus not targeted by the vaccine. If we see these antibodies even in vaccinated people, it would suggest they were infected but just didn’t get sick. This will take some time, although it is underway.

In the meantime, basically, we need to assume that vaccinated people can still spread the virus in the same way that other asymptomatic infections would. That is to say, much less than symptomatic infections but not zero.
posted by caek at 1:30 PM on January 11 [3 favorites]

I'm an epidemiologist. Good resources above, but if you need a really pithy answer: duration of immunity and degree of protection against transmission are analyzed in realtime. If transmission is reduced to a measurable degree for, say, 6 months after vaccination, we'll start to pick that signal up in surveillance data shortly thereafter, say ~7 months. We can't guess at these numbers in advance. We pick them up in retrospect. So hang in there, this surveillance is underway and we're all collecting the data very thoroughly.
posted by late afternoon dreaming hotel at 4:54 PM on January 11 [11 favorites]

This is tangential to your question, but if people who have had the vaccine were told they didn't wear masks, anyone not wearing a mask could claim they'd had the vaccine and didn't need to wear one. So until outbreaks actually start getting under control we can't stop wearing masks regardless of what we do or don't know about the vaccine.
posted by capricorn at 6:47 PM on January 11 [1 favorite]

Good article out today, and

There is some indication that vaccination may reduce asymptomatic infection, resulting in reduced transmission. Preliminary evidence from Moderna showed that participants in a clinical trial who received the vaccine and were tested for Covid between their first and second doses had a roughly two-thirds reduction in asymptomatic infections. “That means there’s a substantial, substantial reduction in overall infections just from that first dose,” says Deepta Bhattacharya, an associate professor of immunobiology at the University of Arizona in Tucson.

Experts note the data set was small and more results are needed. Larry Corey, a virologist at Fred Hutchinson Cancer Research Center in Seattle, called the data “suggestive” but said no conclusions should be drawn by limited testing. Dr. Corey is co-leading a federal vaccine testing program doing Phase 3 clinical trials for multiple Covid-19 vaccines and proposed a study on U.S. college campuses that was intended to examine whether Moderna’s vaccine prevents the coronavirus from spreading, but the study stalled due to funding shortages and time constraints.

Here's a a twitter thread from the researcher quoted above to explain his interpretation of the data.

And more evidence from Israel that argues for the vaccine reducing transmission; they tested PCR in everyone exposed regardless of vaccine status, and found a 33% reduction in PCR+ after the first vaccine dose (Pfizer). Less PCR -> less transmission, regardless of whether symptoms develop or not.
posted by Dashy at 10:51 AM on January 12 [1 favorite]

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