Vaccines and communicability.
April 7, 2021 12:37 PM   Subscribe

I wish to know if one can be fully vaccinated and past all the vaccine milestones and still spread a disease. Obviously if I touch a puddle of Covid or breathe in a breath full of Covid I can spread Covid that way, but can my body (in general) still spread it through snot and whatever?
posted by Evilspork to Health & Fitness (12 answers total) 6 users marked this as a favorite
 
Basically, for Covid, would you be past needing masks? Masks as a courtesy gesture would be different.
posted by Evilspork at 12:38 PM on April 7


There isn't evidence one way or the other, really, yet. I have heard anecdotal stories 3rd hand about people getting mild Covid after being fully vaccinated, and spreading it, but I don't know anybody personally who has had that experience so I am skeptical.

My understanding is that it's possible, but not probable.

However, there is a lot still unknown: how the prevalent variants behave (they're known to be more communicable and spread more easily) and whether or not we can actually get herd immunity.

I think we'll be wearing masks for a good while yet. For example, when I go back into the office, I will wear a mask because I don't know how many of my coworkers might have people at home who can't be vaccinated. I think we'll be doing that for a longer period than we'd like to.
posted by pazazygeek at 12:45 PM on April 7 [2 favorites]


There is some new good evidence that the mRNA vaccines actually do prevent asymptomatic spread.
posted by fancypants at 12:49 PM on April 7 [12 favorites]


The data that I've seen so far seems to indicate yes, you can give COVID to someone else if you're been fully vaccinated.

The chances are less than if you're unvaccinated, because a: you're less likely to catch it in the first place, b: your viral load is lower (because your body is successfully fighting and removing some of the virus), and c: because your symptoms are likely to be less severe (so less sneezing/coughing, less need to come in close contact with medical providers, etc). But if you catch COVID after being fully vaccinated, you can still give other people COVID. Especially right now where in most public situations where there are a mixture of vaccinated and unvaccinated people around, it's still important to wear a mask.

(My university has been doing some epidemiological modeling all the way through this and has done a surprisingly good job of predicting and curbing spread. They recently shared that based on this modeling, if over 50% of students are vaccinated, the risks of in-person learning are low enough that we can do it-- but only if everyone continues to wear masks until vaccination rates are higher. It takes into account what we know about the transmissibility from vaccinated folks.)
posted by tchemgrrl at 12:56 PM on April 7 [7 favorites]


My reading of the current situation is that scientists will always say "it is possible" until you prove that something is categorically impossible. Proving that something is truly impossible is difficult and takes a long time.

This has been a big problem throughout the COVID pandemic. There have been many times when it is pretty clear that "x" is true, but public health officials refuse to say that because it hasn't been categorically proven yet, and so we end up with a muddled message that the public doesn't understand and the result is death and inconvenience. The initial version of this problem with with masks in the US.

With that introduction, my reading of the current evidence is that it might possible to spread COVID after being fully vaccinated, but it is exceedingly unlikely, to the point that you're better off worrying about getting hit by a car on the way to your vaccinated friend's house instead of worrying about getting COVID from her. So, from a practical standpoint, you can act as though people who have been fully vaccinated cannot spread COVID.
posted by Winnie the Proust at 1:17 PM on April 7 [14 favorites]


The evidence isn't complete yet, but so far testing indicates that vaccinated people are also highly unlikely to spread the disease to the unvaccinated. It's looking good enough that I'm thinking about indoor dining again. But you can see in the article I linked the ultraconservative approach Winnie-the-Proust describes, which I think is actually unhelpful in communicating with the general public.
posted by praemunire at 2:14 PM on April 7 [3 favorites]


You can be an asymptomatic carrier of a disease, but it's extremely rare. [....]
Until this particular episode it's been written in stone that a vaccine, to a very high order of probability, prevents you from getting the disease you were vaccinated against


Nah, it's always been a numbers game and depends on the disease and the vaccine. SARS didn't seem to show asymptomatic or presymptomatic spread, COVID-19 shows both, oops. And all vaccines have finite efficacy. It's not at all a ridiculous question to ask which measures stack to a certain level of control, and the answer is not preordained, esp. as the mix of variants shifts.

Remember, the vaccine trials were measuring primarily symptomatic and severe disease. Moderna also took PCR swabs, and more information is now being collected, to refine the picture.
posted by away for regrooving at 2:16 PM on April 7 [5 favorites]




Until this particular episode it's been written in stone that a vaccine, to a very high order of probability, prevents you from getting the disease you were vaccinated against, and thus protects the people around you because you can't spread a disease you can't get. You can be an asymptomatic carrier if you have the disease, but by definition you can't spread a disease you're immune to.

That's simply incorrect. You're conflating illness (you feeling physically sick) and the ability to infect others (the virus replicating in your body in a way that lets it spread to others), and not all vaccines prevent both things equally well. Perhaps most infamously, the vaccine for Marek's disease in chickens prevents illness but not contagion: birds vaccinated against Marek's disease do not get sick, but the virus is still replicating in them, and they can still infect other birds (vaccinated or not). Most human vaccines prevent both illness and disease transmission. We've been able to pick and choose the best vaccines over decades, and/or we've been lucky and the earlier vaccines we've developed for those diseases happened to be able to prevent both. But even among human vaccines, not all quite match the model you're describing. For example, there's some research pointing towards asymptomatic infection and transmission among people vaccinated for pertussis/whooping cough with the acellular vaccine (though that's almost certainly only one of a bunch of factors behind its resurgence in recent years).

Note that this is all somewhat distinct from the question of whether or not you can end up getting sick despite being vaccinated (broadly the scenario tchemgrrl is describing). All vaccines do have some sort of failure rate, but in cases like the vaccine for Marek's disease, the risk of falling ill and the risk of getting infected and being able to spread the disease to others are decoupled, while in the case of, say, the measles vaccine, the risks of getting infected, falling ill, and spreading the disease are all tied together, and are all affected similarly by vaccine efficacy.

In the case of COVID-19 specifically, we know that SARS-CoV-2 absolutely can spread via pre- or asymptomatic carriers, and assuming that this wasn't happening was one of the things that allowed it to spread so far and so fast initially. Additionally, a number of the earlier clinical trials were monitoring for (any) COVID-19 symptoms and confirming the presence of SARS-CoV-2 with PCR tests, but were not giving frequent PCR-based tests to monitor for asymptomatic infection of healthy clinical trial volunteers, so they had no way of analyzing whether or not asymptomatic infections (let alone disease transmission) were being prevented. Between those two factors and the fact that we know that not all vaccines prevent both symptomatic illness and asymptomatic infection and transmission, it absolutely made sense to be cautious until we got actual data, because we couldn't assume that lack of COVID-19 symptoms post-vaccination meant lack of SARS-CoV-2 transmission. Further investigation (including examination of the Moderna clinical trial volunteers who got more frequent PCR tests, which we can use to measure asymptomatic infection rates) is increasingly supporting the conclusion that we've lucked out and these vaccines (or at least the ones available in the US and Europe, for which we have a lot more publicly available data) do prevent not just illness but also asymptomatic infection, and with it asymptomatic transmission. This is super exciting and an immense relief! But this outcome was not guaranteed, and having to walk back guidance about this sort of stuff can be a public health nightmare in its own right. (Of course, this is exactly why scientists and public health officials take the "ultraconservative" approach of calling things unlikely but not impossible until the evidence is pretty strong.)
posted by ASF Tod und Schwerkraft at 4:52 PM on April 7 [9 favorites]


There is good evidence that the mRNA vaccines are around 90% effective at preventing any detectable infection, which includes PCR positive with no symptoms. There’s also evidence that viral load is around 1/3 to 1/4 as high when vaccinated people do get infected, though how that corresponds to infectiousness isn’t as clear. In any case, that’s only for the 10% not covered above.

People who are still saying “we don’t know whether vaccines prevent asymptomatic spread” are just behind the facts at this point. For what it’s worth, my impression (mostly from listening to TWIV) was that it was always fairly unlikely that the vaccines wouldn’t reduce asymptomatic spread at all. The question was just whether it was going to be a little or a lot, and now we know the answer seems to be “a lot.”

I do agree there is a tension in communicating this stuff between being completely rigorous in terms of what we do and don’t know, and giving people an accurate intuition about what the most likely outcomes are. Also, early in the pandemic there was so much underselling from official sources that I think some people have gotten used to mentally amplifying the potential risks, which is not as adaptive now that the Trump admin is over and there seems to be legitimate good news on some fronts. The main thing that still personally worries me is opening up before enough people have been vaccinated, not the effectiveness of the vaccines themselves.
posted by en forme de poire at 5:47 PM on April 7 [7 favorites]


Practically speaking, the odds of you getting COVID when vaccinated with a high enough viral load to pass it to someone else are very, very low. We should not need masks indefinitely once vaccination rates are high and community spread is very low. There are examples of calculations you could make here to understand the risk (which must be compared to the risk of other terrible things to think about in context, like dying in a car accident, dying from the flu, etc):
calculator
posted by ch1x0r at 6:36 PM on April 7 [1 favorite]


It took literally over 15 years for HIV researchers to publicly admit that having no virus in your blood due to medication-based suppression is a very similar state to having no virus in your blood because you don't have HIV. IOW, if you have HIV and are on medications to suppress the virus to an undetectable level, you do not transmit HIV because you don't have any to transmit.
posted by OmieWise at 2:47 AM on April 8 [4 favorites]


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