This question is really about all contagious diseases, but inspired by covid and could just as well be limited to covid. We don’t know whether being fully vaccinated prevents a person from being a spreader of the disease even if they feel no symptoms. I just can’t picture what the mechanism might be. Is the fear that they would be facilitating the virus multiplying and shedding? Why aren’t the ability to host virus multiplication, and the ability to get sick from doing it, both controlled by vaccination? What is actually going on in the body of a hypothetical immune spreader?
The term is ‘asymptomatic shedding’ and it’s a concern for many other infectious viruses. Vaccines don’t completely block viruses from infecting cells; they just prime the immune system to defeat them before they multiply enough to cause illness.
That means you could get vaccinated and have a small population of viruses reproducing in your respiratory system and aerosolizing in large enough numbers to infect someone else, but not enough to make you really sick. AIUI that’s the tough thing about COVID, it’s not a super durable virus but it’s incredibly good at catching a breeze and staying airborne.
So I’m assuming this is the reasoning behind the medical (as opposed to social) requirement that vaccinated people are still required to wear masks in public?
Exactly.
Lots of nice info in this article.
- Don’t vaccines stop infection and thus transmission?
Some do and some don’t. The gold standard in vaccinology is to stop infection as well as disease – providing so-called sterilizing immunity. But it’s not always achieved. The vaccine for measles, for example, provides it; the one for hepatitis B does not.
- Do Covid vaccines have to prevent infection to stop transmission?
Not necessarily. To the extent a vaccine prevents infection, it also prevents forward transmission. But it can do the latter without doing the former. Since SARS-CoV-2 spreads through respiratory particles from an infected person’s throat and nose, a vaccine that reduces the duration of the infection, the amount of virus in the respiratory tract (the viral load), or how often an infected person coughs may decrease the likelihood of it being transmitted to others.
I don’t have a study to back this up, but aren’t something like <20% of people super spreaders who cause most of the contagiousness of this virus? If so, I wonder what happens with them when they are vaccinated.
To be clear, the COVID vaccines probably do prevent transmission, because that’s how almost all vaccines work. But the scientists say they don’t know, because they’re not completely sure, because that wasn’t the primary focus of the studies, and the media takes that and runs with it, because panic sells, and so you get people just taking it for granted that the vaccines don’t stop transmission.
Is it practically certain that vaccination reduces transmission, but possible that it does so anywhere from almost 0% to almost 100% of the time? In other words, likely not a binary.
Not all vaccines prevent transmission. That requires sterilizing immunity and not all vaccines provide that. They do reduce transmission because the person will have lower viral loads for a shorter time and they won’t be coughing or secreting or rashing or whatever the pathogen uses to transmit to other people.
Luckily, we’re starting to get more data that shows that the Covid-19 vaccines are doing a great job at severely reducing transmission. This is best measured by regularly covid testing (which wasn’t done in all the vaccine trials) in the short run. In the long run, we’re simply seeing less transmission in contact tracing. It’s great news!
I mean, is there any realistic scenario where a vaccine lowers viral loads by enough to be as effective in preventing symptomatic disease as the various Covid vaccines have been found to be without that drop in viral load greatly diminishing transmission? It seems to me that it would be really, really weird for your immune system to be primed enough to almost completely eliminate symptomatic disease, but not enough to prevent you from shedding like a Siberian Husky in July.
Exactly. The other thing that has been misunderstood/misrepresented in the press is the idea that the vaccines were not “designed” to stop transmission. Stopping disease and stopping transmission are not mutually exclusive design objectives - quite the opposite, the way you design a vaccine is exactly the same to achieve both objectives. But transmission was not the efficacy criterion of the studies simply because it’s much more difficult to get data on transmission. If you have a study with 30,000 people, they are not imprisoned in a lab, they are going about living their lives as normal. It’s easy to monitor them to see if they get sick. But how would you measure whether they are infecting the millions of other people that they interact with who are not in the study?
If you have a disease like Ebola, where transmission is quite low simply because it debilitates and kills you so fast, then it is possible that a vaccine that ameliorates disease might actually increase transmission. It could allow more cases of asymptomatic or mildly symptomatic disease, where most infefted people are now well enough to keep walking around and interacting with other people, while still shedding virus.
But in the case of SARS-CoV-2, we are starting with a baseline of high transmission because it is already the case that most infected people are asymptomatic or mildly symptomatic and can continue interacting with others spread it to a lot of other people. So I think you are correct for SARS-CoV-2 - it seems extremely unlikely that a vaccine that “works” in any sense in priming your immune system to attack the virus more effectively would not reduce transmission at least to some degree.
Point taken with regards to viruses that never got the “killing your hosts too enthusiastically can be counterproductive” memo like Ebola. I hadn’t thought of that sort of mechanism.
It’s essentially the same sort of mechanism that’s involved when people talk about viruses naturally evolving to become less virulent.
Yes, as I stated in my post. Data is starting to support significantly lower transmission.