Seek clarification re covid vaccine and spreading the virus
June 27, 2021 11:32 AM   Subscribe

CDC says, "We are still learning how well vaccines prevent you from spreading the virus." What does this mean, exactly?

I have an acquaintance who will not get the vaccine because she says it was developed too quickly. (An aside: I'm curious to know how long she would have needed the vaccine to be studied before she would feel secure receiving it, and what criteria she would go by to determine this. Ho-hum.)

I told her that part of the reason I chose to be vaccinated was because I felt a moral obligation to at least try to protect my fellow human beings. She countered that the vaccine does not keep one from spreading covid to others.

The CDC website says, "We are still learning how well vaccines prevent you from spreading the virus that causes COVID-19 to others, even if you do not have symptoms. Early data show that vaccines help keep people with no symptoms from spreading COVID-19."

I'm not sure how to interpret the CDC statement. Does it mean that you can only spread covid if you actually get covid? If covid reduces your chances of getting the virus by, like, 95% in the first place, then wouldn't that mean that if you were vaccinated you would only have a 5% chance of spreading the virus if you were to get covid vs someone who is not vaccinated, who would have, like, a 100% chance (or so) of spreading the virus if they were to get covid. Or does the vaccine actually not prevent covid at all, but just prevents covid symptoms 95-ish% of the time?

I've googled my head off and haven't been able to find a specific answer to my questions.
posted by SageTrail to Health & Fitness (6 answers total) 2 users marked this as a favorite
What they're saying is that there hasn't yet been a conclusive study published that specifically proves that the COVID-19 vaccines stop you from spreading the virus. There are many people in the scientific/medical community who believe that this is an overly-cautious and wishy-washy statement for the CDC to make.

When you test a drug like a vaccine, you pick a specific definition of "success" *before* you start the study - this is because if you're allowed to pick a definition of success after the study has begun, it gets really easy to fudge things with statistics. The initial efficacy studies that were required for provisional FDA approval used "does this reduce people's chances of getting severe COVID-19 disease?" as their success endpoint, because 1) that's a really important endpoint! and 2) it's relatively easy to keep track of.

Placebo-controlled trials like these are the gold standard for medical interventions, but they're not suitable for studying all types of interventions and all types of endpoints. We're not likely to see many (any?) additional placebo-controlled trials of the existing vaccines because now that we have seen that the vaccines work it would be unethical to not provide a real vaccine if we had the ability to do so. (There might be tests of different vaccines against each other, though, for instance.)

And something like "how well does this vaccine protect against the vaccinated person transmitting to others" would be an incredibly difficult endpoint to study, because we don't have perfect contact tracing and we don't always know who infected whom. Even "how well does this vaccine protect against asymptomatic infection" is harder to test than the "severe disease" endpoint - you need the test subjects to get regular tests.

It's my understanding that, given what we know about how vaccines work, and how coronaviruses are transmitted, there are lots of good reasons to think that being protected against severe disease will also protect others who come in contact with you, and that is now being borne out in the data as we study patterns of transmission in vaccinated, unvaccinated, and mixed groups of people.

So, short answer: there are lots of good reasons (both theoretical reasons and new data-backed reasons) to believe that the FDA-approved vaccines vastly reduce your chances of transmitting COVID-19, but as the CDC says, we really are "still learning how well vaccines prevent you from spreading the virus that causes COVID-19 to others." When will the CDC say that we've learned enough for them to say something actually useful and actionable about whether vaccines prevent transmission? I guess we're still learning about that as well.
posted by mskyle at 11:56 AM on June 27, 2021 [8 favorites]

Re: it being developed too quickly, worth pointing out to her that the mRNA technique's been worked on since the '80s, if that helps.
posted by metabaroque at 12:01 PM on June 27, 2021 [4 favorites]

Best answer: Here's an explanation. Vaccine trials focused on whether the vaccines prevented symptomatic infection, not asymptomatic infection. But subsequent studies suggest the vaccines are also good at preventing asymptomatic infection. If people don't get sick, they're not going to spread the disease. When people do get sick, vaccines also seem to reduce the viral load, which should reduce the risk of infection.
posted by pinochiette at 12:06 PM on June 27, 2021 [11 favorites]

the reason I chose to be vaccinated was because I felt a moral obligation to at least try to protect my fellow human beings. Thank You for believing in facts and making the right choices.

There are a few post-vax breakthrough cases of Covid, in which case, the virus can be spread. in my area, there have even been a few deaths of people who were quite high-risk. But the vax does seem to be 98+% effective at stopping people from getting Covid at all, meaning those who are vaxxed are only very rarely able to spread it. All the data indicates that vaccination is very effective at stopping the spread. New cases are mostly in areas/populations with low vax rates.

I'm generally a skeptic, but Public Health folks are generally trustworthy (despite what Trumpists might say) and they are far better able to make sense of the data. Your friend is risking their health, and others.

I just read that only 4% of Japanese are vaxxed, that Viet Nam does not have vaccine available. Americans are lucky; vaccine is widely available. It's tragic that so many are declining the opportunity.
posted by theora55 at 12:23 PM on June 27, 2021 [1 favorite]

The point of the vaccine is to prime the immune system to recognize and disable the virus. If it's doing that, the virus isn't going to be able to reproduce anywhere near as fast as if it wasn't doing that. Therefore, a vaccinated person's maximum viral load can reasonably be expected to be lower than an unvaccinated person's. Therefore, a vaccinated person can reasonably be expected to spread the virus at a lower rate than an unvaccinated person.

Where the uncertainty comes in is that nobody has much of an idea of the actual numbers attached to any of these likelihoods. Maybe a vaccinated person is ten times less likely to be a spreader than an unvaccinated person. Maybe only half as likely. Maybe a hundred times less likely. Nobody knows. But the way vaccination as a thing works means that we can be pretty sure there would be a significant reduction in personal infectiousness.

Your friend's claim that the vaccination does not keep one from spreading the virus to others is probably true, strictly speaking; but it's true in much the same way as claiming that wearing seat belts doesn't keep people from being killed in car crashes. People who insist on reasoning in absolutes and binaries rather than thinking about relative likelihoods are generally very bad at evaluating risk in any useful way.
posted by flabdablet at 12:32 PM on June 27, 2021 [8 favorites]

I got vaccinated.

I got covid after being vaccinated.

I warned everyone that I had been in contact with that I had contracted covid-19. The people that I was in brief or incidental contact with did not contract covid-19 from me.

My wife, who had been vaccinated for months refused to leave the house. Six days after I was diagnosed, she began to show symptoms and came down with covid

This is anecdata, obviously. I attribute my breakthrough infection to my weak immune system. This is not the first time this has happened - as a child I contracted mumps through brief contact at school even though I had had the MMR vaccine.

Logic suggests that because my wife had had the same contacts that I had had, and was not infected concurrently with me, that she contracted it from me. The logic is further supported by the fact that my home is a closed system with windows that are painted shut, so despite best efforts she was living in a soup of shed virus that eventually overcame the protection the vaccine provided.

I know that logic is a good way to go wrong with certainty, but this is the story I tell myself. Anecdata, yes. Ymmv.

All that said, I do believe that the vaccine attenuated the disease course for me and my wife. We both have comorbidities, especially lung and heart problems that may very well have led us both to a much more devastating course of illness had our bodies not been primed to fight.
posted by Vigilant at 1:03 PM on June 27, 2021 [13 favorites]

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