Post #959: If you’ve been vaccinated, do you still need to wear a mask?

Posted on January 17, 2021

If you’ve been vaccinated, do you still need to wear a mask?  I got this question from a reader a couple of days ago, and gave a partially-correct answer via email.  Here, I’m going to post what I believe to be the fully-correct answer.

Briefly:  Yes.  The CDC says you still need to wear a mask, social distance, and so on.  As long as the virus remains widely circulating in your community.  That’s not just the CDC being its usual fussy self.  There are sound reasons for that, involving non-trivial risk of catching and spreading disease.

In this post, I’ll try to explain that.  But, at the minimum, that “90% efficacy” you read about doesn’t include asymptomatic or mildly symptomatic infections.   They only counted severe, symptomatic cases.  The reduction in total coronavirus infections is unknown, and you can still spread the disease if you get a mild case of it.  In addition, that 90% reduction was based on all the participants maintaining their existing COVID hygiene.  If you get vaccinated, then start hanging around maskless in bars, you plausibly have greater odds of getting infected after the vaccine than you did before it.

But surely that has to prompt a few follow-on questions.  I’m not going to provide detail on these, in this post.  But I’ll give you my answers.

Well then, when can we  ditch the @#$@# masks?  I think the answer to that is “when the Governor says we can”.  We’ll get there when we get there.  I don’t think it goes any deeper that that.  It really shouldn’t be an individual-level decision, but that’s a topic for another post.

If you’ve had COVID-19 already, do you still need to get vaccinated?  Again, the CDC says yes.  But I think that’s an inefficient and illogical recommendation.  We’d get a bigger reduction in cases if those who have already had COVID go to the end of the line for vaccines.


What’s the deal with getting just one shot, instead of two?  That’s been kicked around, and that’s an easy answer:  Where two shots gives you 90% efficacy in preventing sever COVID infections, one shot gives you just over 50% efficacy.  And so, from a public health standpoint, as long as vaccine remains scarce, that debate is about whether you’d be better of with N individuals with high immunity (90-ish %), or 2N individuals with lower immunity (50-ish %).  A reasonable person could go either way on that.

As you may have already figured out, those last two questions are really two forms of the same question.  Do you want N individuals with super-immunity (both infected and vaccinated;  vaccinated with both shots), or N+M individuals with less-than-super immunity (either infected or vaccinated, vaccinated with just one shot).

In both cases, the CDC comes down on the side of fewer people, with greater immunity.  Maybe they should split the difference and give those who are known to have been infected just one shot.  But given the trouble they’re having just getting people vaccinated, it seems unlikely that our vaccination protocols could deal with that level of sophistication.

If the pandemic is over by the time my vaccine dose arrives, should I still take it?  Unsurprisingly, the CDC hasn’t addressed this one, but my firm opinion is, yes COVID-19 probably is not going to disappear as the 1918 Spanish Flu did.  Experts predict that COVID-19 will become endemic, just part of a long list of viruses that circulate to some degree in the population.  Not only will you need to get vaccinated, you’re probably going to need to get periodically re-vaccinated for the rest of your life, to reduce your odds of catching it.  Kind of like seasonal flu on steroids.

Details follow.  As I said, I’m only addressing the first question below.

Background, definitions, and analysis:  What does 90% efficacy mean?

The main point of this section is that nobody knows the extent to which vaccinated individuals may still have mildly symptomatic or asymptomatic COVID infections.  And you’ll never know whether or not you are fully protected against severe COVID, or still bear some risk of having severe COVID, after you are vaccinated.  And the combination of the two means you’d be well-advised to keep wearing a mask, after vaccination, for your own sake as well as for the sake of those you are in contact with.

By vaccinated, I mean fully vaccinated.  At least two weeks past your second dose of vaccine.  That’s the time required for full development of antibodies.  Before that, you’re only half-vaxxed, so to speak, and you’d continue to wear your mask for your own protection, as you do now.

My point is, I’m not talking about the time between your first shot and full development of antibodies.  It’s about whether you need to wear a mask after that, once you’ve developed whatever antibodies you are going to develop.

What do they mean when they say the COVID-19 vaccine is 90% effective, and how did they test that?  This seems like a picky technical question, but it goes to the heart of why you should maintain your COVID-19 hygiene even after vaccination, as long as the virus is widely circulating in your community.

Let me outline the method from this scholarly publication , more-or-less the same as in this scholarly publication, putting key elements in boldface.

To test the vaccine, they randomly assigned volunteers to receive either the vaccine or a placebo.  Because half got the placebo, the participants carried on with their lives as they had been, including maintaining mask, social distancing, and other COVID hygiene.  None of them could count on becoming immune.  They then counted the number of people who had at least two symptoms from a pre-defined list of severe COVID symptoms, and tested positive for COVID-19 with a nasal swab (PCR) test.  It is unclear whether those individuals sought medical attention, or were screened by the researchers, but based on internet chatter, I think that’s among individuals who sought medical attention.

And then the researchers counted cases.  They found that there were about 10 symptomatic, diagnosed COVID infections in the placebo group for every one in the vaccine group.  From that, they concluded that the vaccine reduced symptomatic COVID infections by 90%, relative to the placebo group, while both groups maintained existing COVID-19 hygiene

And that’s what they mean by 90% effective.  To be clear, that doesn’t mean that the vaccine reduced total COVID infections by that amount.  And, again to be clear, it doesn’t mean that it reduces your odds of any COVID infection by that amount.  They actually don’t know the extent to which total infections were reduced.  The vaccine might have worked by preventing all levels of infection.  If might have worked by preventing no infections, but greatly reducing severity of illness.  It probably worked from some combination of those two.   But at this point, nobody knows what combination.  I understand it, they are now in the process of going back, taking blood samples, and trying to estimate that.

As an aside:  This is not how vaccine effectiveness is normally measured.  Vaccine effectiveness is normally measured by the ability to prevent all infections, symptomatic or not.  For example, that’s how flu vaccine effectiveness is measured, so that individuals who show up with antibodies for flu in their blood, but no symptoms, are counted as vaccine failures.  When we say that the best flu vaccine is 60% effective, that can’t be directly compared to the COVID-19 vaccine, because the COVID effectiveness number doesn’t count (an unknown number) of asymptomatic or mildly symptomatic cases.

Well, so what.  Does this matter?  How many people plausibly had mild or asymptomatic infections?

Answer:  There was a vastly larger pool of individuals who had some symptoms of respiratory illness, but did not meet the criteria for being screened for COVID.  (That is, did not have at least two of the serious symptoms as specified by the research protocol.)

What do I mean by “vastly larger”.  If you want to know, read this critique,  bearing in mind that the researcher who wrote that has a reputation for having an anti-vaccine bias.  That said, the numbers are what they are.  I’m just going to quote from that critique, to show the numbers, emphasis mine

"All attention has focused on the dramatic efficacy results: Pfizer reported 170 PCR confirmed covid-19 cases, split 8 to 162 between vaccine and placebo groups. But these numbers were dwarfed by a category of disease called “suspected covid-19”—those with symptomatic covid-19 that were not PCR confirmed. According to FDA’s report on Pfizer’s vaccine, there were “3410 total cases of suspected, but unconfirmed covid-19 in the overall study population, 1594 occurred in the vaccine group vs. 1816 in the placebo group.”

And that doesn’t even address the potential for totally asymptomatic infections.

In other words, we know that the vaccine is pretty good from the standpoint of preventing severe COVID infections.  We don’t really know, yet, how good the vaccine is at preventing all cases.  And that differences is due to the potential for (numerous) mild and asymptomatic COVID infections in the vaccinated population.  Those aren’t part of the “90% efficacy figure”.

Second, look what happened to the measured efficacy of the Chinese vaccine.  Under research conditions, it was 75%.  In the real world, it was more like 50%.  That was attributed to giving the vaccine to a heavily-exposed population (health care workers).  Now, the Chinese vaccine is a different type — it’s a killed-virus vaccine, it’s not an MRNA vaccine like the Moderna and Pfizer vaccines.  But that’s still a big difference between theory and practice.

The US CDC is collecting the data to assess the real-world efficacy of the Pfizer and Moderna vaccines.  Eventually, they’ll be able to say something about how well they worked at preventing infection in the field.

But for now, you’d be well advised to take that 90% efficacy number with a grain of salt.

One more concept: Perfect versus leaky vaccines, and Schrödinger’s cat.

I keep saying that the vaccine is 90% effective (at something).  But does that mean that 9 out of 10 get complete immunity, and one is a dud?  Or does that mean that each of us still has roughly 10% the chance of getting infected, and nobody is completely safe?

The first case is a “perfect” vaccine.  When it protects you, it fully protects you, no ambiguity about it. The second is a “leaky” vaccine, which never fully protects you, just reduces your odds of infection somewhat.

In general, most human vaccines are thought to be perfect vaccines.  Or very close to it.  They might not protect everyone who gets the vaccine.  But it’s basically binary.  You’re either protected or not.

My reading is that, probably, the COVID vaccine is a perfect vaccine, but we may not ever know that.

The more important point is that you’ll almost certainly never know about you, personally.  In theory, they could draw blood and test for antibodies after you are vaccinated.  If you show a high antibody titer against COVID, you’re probably immune.  But nobody is going to do that for you, so you’ll never know.

The upshot is that, in a practical sense, you’re going to have to treat yourself as if this is a leaky vaccine.  That is, as if you still have 10% of the chance of getting COVID that you had before being vaccinated.  Not because this truly is leaky, but because you have no way of knowing if you’re in the 90% that’s fully protected, or the 10% that’s not.

In effect, you become the COVID equivalent of Schrödinger’s cat.  You are both immune and not immune, and you won’t really know which until you get sick.


Why does CDC want you to keep wearing a mask after vaccination?  And why should you?

So with that as background, let me lay out the reasons that you should keep wearing a mask, even after being vaccinated.

First, you have reduced your chances of severe COVID-19 by 90% if and only if you maintain your current COVID hygiene.  You still have one-tenth the chance of picking up a severe case, compared to your odds before you were vaccinated.  Assuming you change nothing else.

If you couple that vaccination to an increase in risk-taking behavior, you could easily offset (some, most, or more-than-all of) that 90% reduction.  E.g., if you go from being a shut-in, pre-vaccine, to hanging around maskless in bars, post-vaccine, chances are that you’ve upped your odds of getting infected.

My point is that the 90% reduction isn’t some absolute number.  It’s relative to your current odds.  If you do other things to change your odds of infection, that matters.

Example:  Suppose that wearing a reasonable mask cuts your odds of getting COVID-19 by two-thirds (66%).  If you go from wearing a mask, un-vaccinated, to bare-faced, vaccinated, your odds of infection are now (.10/.33 =)  30% of what they were.  Under those assumptions, your change in behavior offsets one-third of the protection provided by the vaccine.

Second, you may still be able to spread disease once vaccinated.  If you drop the mask, that makes you a much more efficient spreader of disease.

Whether or not this matters materially depends fairly critically on the incidence of mild-to-moderate infections among the vaccinated population.  If there are a lot of those, then you’re as good a disease vector after vaccination as you wee before.  Presumably, at some point, researchers will settle that question.  But until then, you (and the CDC) have to assume that you could be a COVID carrier.  Just as you could be a carrier now, prior to being vaccinated.

Example:  Suppose that the main effect of the vaccine is not to prevent infection at all, but merely to convert more infections to mild or asymptomatic infections.  Assume that the risk of transmitting disease to others triples when un-masked, compared to masked.  If everybody who is vaccinated then takes off their masks, under those assumptions, you’ll actually see an increase in the spread of disease with vaccination and no mask, compared to mask and no vaccination.