Today’s post is all about post-vaccine behavior. In particular: how do things change now that some people have the vaccine, but not all. As of this writing, the NY Times reports 6.7 million people have been vaccinated. The Biden Administration is planning a more aggressive approach, which may speed things up. But: as I’ve said before, it is going to be many months before we will be frolicking unmasked in the meadows or, you know, close talking to strangers at a bar. So the questions pile up: in the intermediate period, what can we or should we do?

Before I get into this, a few quick resources. First, here are my two earlier posts on vaccines (overall & pregnancy). Second, I highly recommend this episode of the Bio Eat World podcast if you want to hear more about the Moderna vaccine, how it was developed and produced. It’s not going to answer these behavior questions, but it’s a great explainer on how these vaccines operate and frankly just how amazing the science is.

The Questions

My wife was vaccinated at work, but I’m not, and the kids aren’t. Is it safe for her to go out unmasked? Or should she still keep masking when she is out so she doesn’t infect us?

My 90-year-old grandmother got the vaccine. Yay! Can she hold her great grand-daughter now? We are dying to introduce them.

I got my second dose!! So did some other friends. Can we all hang out together without masks now?

I actually HAD COVID-19 months ago and I tested positive for antibodies. Is it as if I have the vaccine? Can I hang out unmasked with vaccinated people?

I could go on. But you get the drift. What do vaccines let you do, and what do they not? And beyond that, is vaccine immunity the same as having-had-COVID immunity? Better? Worse?

As usual, I’m going to argue that there are some main underlying principles that will help us answer all of these questions. So let’s dive into those, and then we can come back to some specifics. (Thanks to the COVID-Explained Expert Team for some help here, notably Lindsay Shultz, Susan Johnson and Galit Alter).

Can Vaccinated People Spread COVID?

This is really the big question and I think it’s a confusing one. For many people, the idea is: you’re vaccinated, so you aren’t getting sick. So, how could you spread the virus?

The answer is that the vaccine preps your body to be ready to fight the virus if it is introduced. This means it protects you from getting sick from the virus. The vaccines we have are doing a great job of that. But vaccines do not necessarily protect you from spreading a virus because they don’t prevent you from being infected. In other words, it is possible you’d have asymptomatic infection and pass it on.

This is not just a comment about COVID-19 vaccines. Many or most vaccines have this feature. They provide effective immunity, meaning you do not get sick, but not sterilizing immunity, meaning you can possibly infect others. This is true of the flu vaccine, for example. There are vaccines which give sterilizing immunity, for example HPV and the measles vaccine, but it is not universal by any means.

We do not yet know details about whether and how much COVID-19 vaccination limits your ability to spread the virus. To be clear, there is a lot of reason to think that it would limit your ability to spread at least to some extent. Notably, asymptomatic infections seem to spread less than symptomatic ones. Evidence suggests that asymptomatic people are perhaps 75 or 80% less likely to spread the virus than symptomatic people (see a summary in Nature here, and this meta-analysis on household transmission).

This means the fact that vaccination eliminates symptoms in most people will also mean it reduces spread a lot. Of course, if vaccinated people take fewer precautions, some of this reduction will be cancelled out. Obviously if we could be confident that the vaccine is preventing infection, not just illness, that would be game-changing. But we do not know this yet.

When will we know more about this? The COVID-19 vaccine trials have as their primary endpoint illness with COVID-19. But the trials are also collecting secondary endpoints which will help understand whether they block infection. This includes collecting swabs to look for evidence of virus in the upper respiratory tract, and also testing for evidence of antibodies to other part of the virus not targeted by the vaccine. If we see these antibodies even in vaccinated people, it would suggest they were infected but just didn’t get sick. This will take some time, although it is underway.

In the meantime, basically, we need to assume that vaccinated people can still spread the virus in the same way that other asymptomatic infections would. That is to say, much less than symptomatic infections but not zero.

Is Vaccine Immunity the Same As Having-Had-COVID Immunity?

The technology through which the mRNA vaccine works in your body is basically exactly how your body generates natural immunity (seriously, go listen to the Bio Eats World podcast). There is one difference. When you are infected with the actual COVID-19 virus your body produces antibodies that target multiple parts of the virus, not just the spike protein (although the spike protein is often a primary target). The vaccine targets only the spike protein.

The vaccine generates a really robust antibody response to the spike protein which could actually be more effective than natural immunity, which is why generally even people who have had COVID-19 are recommended to get the vaccine. “Natural” immunity may be more protective against large mutations in the spike protein, but (a) this isn’t obvious since natural immunity also often targets the spike protein and (b) it requires you to get COVID-19 which is not good.

However: from the standpoint of these behavior questions, it is broadly reasonable to think of both naturally acquired immunity and vaccine-acquired immunity as the same. That is: the possible risk of spreading the virus to others remains, but you are yourself highly protected from getting sick (or, sick again in the case of natural immunity).

Importantly, it is possible that immunity wanes over time and this is true for both naturally and vaccine acquired immunity and we do not yet know the time frame.

Back to the Questions

So let’s get back to the questions. I’ll try to bucket them a bit.

One adult is vaccinated in the household. They should still be careful outside of the household to avoid infecting other people at home.

Elderly relative is vaccinated, can we see them? This is more complicated. In a lot of cases (like the 90 year old question above) the primary (not only, but primary) reason not to see them was to avoid getting them sick. We do not want anyone to get COVID-19, but the risks are much, much greater to the elderly. And there are clear benefits to seeing each other. So I would say that at a minimum the vaccination changes the calculus a lot here. You cannot completely rule out that the older person will infect others, but it is also a lesser concern. Especially given the fact that they will not have symptomatic infection, the risks are low.

It’s also often the case that the older people in these examples are themselves fairly isolated and not doing a lot of risky activities. The main risky activity might be seeing their family. In which case, great. Now that risk is gone for them, you should feel much more comfortable.

Can I have an unmasked indoor party with my other vaccinated friends? What about the ones that already had COVID-19? Early on there was some discussion of how, basically, saying yes to questions like this would encourage young people to go out and get COVID-19 so they could do this. So I am reluctant to answer this. But the answer is sort of yes or, at least, that such a party would not lead to you all getting sick.

HOWEVER: see all discussion above. If you get infected at the party, even if you do not get sick, you could bring it back to your family. So unless you’re literally living in a bubble where everyone is vaccinated the most careful thing is to not having any indoor unmasked parties for now.


I also felt like this writing this. Like, if our kids aren’t vaccinated until the fall can we never do anything? Argh.

BUT: I don’t think it is as bad as all that. For one thing, asymptomatic cases seem to spread much, much less. This means that as more and more people are vaccinated, we’re likely to be able to let down our guard a bit. If we learn there is sterilizing immunity from the vaccine, then that will change things even more.

In addition, when we think about kids, the risks to them are very, very low. If everyone else is vaccinated, we should be able to start to return to normal life even as we wait for children.

Also very important: one of the huge issues we are facing right now that a lot of people are sick. We are running out of hospital beds. People are not getting care for what they need. As we vaccinate people this will improve. Because they will not get sick. This will help us return to normalcy.

So, please cancel your vaccine celebration parties for now, but possibly it’s okay to see your elderly isolated relatives after they are vaccinated. And try to breathe.