Since the earliest reports of the high efficacy of the Pfizer and Moderna COVID vaccines there’s been a significant asterisk attached to that good news. While the vaccines are extremely effective at preventing illness and death – close to full proof on the latter – it wasn’t clear whether they prevented the further spread of the disease. So a vaccine protects you from getting sick but possibly you could still spread the disease to others.
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When I first heard about this possibility in an article by TPM’s Josh Kovensky I was baffled. How could that possibly be true, even logically speaking?
This issue is one of the deepest sources of confusion and inaccurate messaging tied to COVID vaccines.
How is it that a vaccine could prevent you from getting COVID but still allow you to spread it to others? The are a few conceivable ways. But the main one is this. The issue goes back to the structure of the initial human trials. Researchers gave thousands of people either the vaccine or a placebo and then waited to see if there was a difference in the number of people who contracted COVID. But they weren’t testing each person for COVID every week. They waited to see who got sick and then had a lab-confirmed case. Some people might get flagged when they got tested after exposure or just randomly for surveillance purposes. But for the great majority of people they got tested when they showed symptoms of COVID.
But as we know, many people get COVID and never become symptomatic or have such mild symptoms they don’t know to get tested. And we have abundant data suggesting that asymptomatic people can still be very good at spreading the disease.
Here’s what that means. The trials were meant to confirm as quickly as possible that the vaccines were safe and effective at preventing illness. Both are true. But it’s possible that that very high efficacy against illness (well over 90%) was actually just pushing lots of COVID infection below the symptomatic threshold. That’s still great news for recipients of the vaccine. (It’s really important to note that while the ‘efficacy’ is around 95% it’s close to 100% for serious illness and death.) But it could mean that protected vaccination recipients could still be circulating in the population and spreading disease to others.
Most clinicians appear to regard this as somewhere between unlikely or extremely unlikely. But COVID’s a new disease. The stakes are super high. And the initial trial data simply doesn’t address this question.
The uncertainty about vaccines and spread is what’s behind the guidance to keep up your mitigation guard up even after you get vaccinated. But many news reports and even some public health guidance has translated this uncertainty into stating affirmatively that vaccinated individuals can still spread COVID. Not just that it’s a possibility we have to assume until we can definitively rule it out but that it does happen and happens just as easily as before you got your shot. That’s wrong. We do not know that to be true. Indeed, there’s lots of logic, epidemiology and science behind the strong assumption that it’s not true. This creates a huge uncertainty – both actual scientific uncertainty as well as a larger penumbra of confusing messaging – tied to this critical issue.
It is important to note that the vaccines almost certainly won’t be 100% effective at preventing the spread of the disease since we know that they are not 100% effective at preventing illness. But 80% or 95% protection is pretty good individually and at a society-wide level it’s more than enough to crush the pandemic if enough people are vaccinated. The virus just runs out of a critical mass of people to allow it to move through the population. The mathematical chain reaction we know from exponential growth starts working in the opposite direction.
So how do we figure this out?
In recent days two studies have been reported which appear to shed some light on this question. And the news is good.
A study out of the Mayo Clinic, which has yet to be peer reviewed, used retrospective statistical analysis to compare two groups of 31,069 people – one vaccinated and one not – to ascertain how effective the vaccines (both Pfizer and Moderna) were at preventing actual infection. After two doses, they found these vaccines were 88.7% effective at preventing infection itself – not just illness but infection itself. I have no way to judging the quality of the statistical methodology. And again, it’s pre-peer-review.
A separate study out of Israel had similar findings. Israel has the highest COVID vaccination rate of any country in the world. That’s based on a deal struck with Pfizer to allow it to have access to the country’s ministry of health data for research purposes. That study found the Pfizer vaccine reduced actual infection by 89.4%.
There are however more questions about the Israeli study. The study hasn’t actually been released anywhere, not even as an un-peer-reviewed pre-print. Various news organizations got early drafts of the study. The Israeli reporter who says he first reported the study cautions that Ministry of Health sources say the data is being over-interpreted in news reports. Those sources told reporter Nadav Eyal “that the infection reduction data are sketchy at best.” This Bloomberg article, meanwhile, reports that some researchers say the data contains significant bias since people are less likely to get tested once they’re vaccinated. Again, as with the Mayo Clinic study, I’m in no position to evaluate the methodologies or quality of these findings.
Presumably we’ll be learning more in the day and weeks to come.