How many vaccine doses do you need?

Welcome to Plugging the Gap (my email newsletter about Covid-19 and its economics). In case you don’t know me, I’m an economist and professor at the University of Toronto. I have written lots of books including, most recently, on Covid-19. You can follow me on Twitter (@joshgans) or subscribe to this email newsletter here.


When Pfizer and BioNTech announced their encouraging vaccine news last week, they made the following projection:

Based on current projections we expect to produce globally up to 50 million vaccine doses in 2020 …

Given that a full treatment of the vaccine requires two doses, 4 weeks apart, it looked like 25 million people might obtain that treatment initially. Beyond this, they hoped to produce 1.3 billion doses in 2021. That sounds like a lot but it is still a fraction of the global population with 670 million people immunised by the end of next year. Now there will be other vaccine candidates so there will be more production than just that.

The other day, I had one of those naive economist thoughts: if it costs the global economy billions of dollars each day the pandemic continues, not to mention thousands of lives lost, do we really need two doses 4 weeks apart? For instance, why not give everyone their first dose and then, much later, a second dose? You know, we economists …. always thinking about how to lower costs.

So I decided to ask some people who might know something how important it was that people have two doses. Could a single dose do the trick for a time?

We need two doses

I have an answer, it won’t. And I have a limited understanding which I can’t claim to be complete. It comes back to a common theme — immunology is complicated.

Given this, and noting that I may over-simplify and also misdirect, I am now going to put into my own words what the experts were telling me. The issue is how your immune system gets prepared to do the job of identifying and then specifically attacking a particular virus. If the virus is new — ‘novel’ if you will — your own immune system hasn’t seen it. So you need to work it into both recognising the virus’s antigens (the proteins on the virus that a vaccine teaches your immune system to recognise) and then make the right antibodies to neutralise the virus.

Apparently, learning takes time. Your immune system takes a few weeks to make the antibodies to attack a virus’s antigens. So if you have a first infection, you are stuffed which is precisely our current problem. The point of the initial dose is to give your body a ‘light’ introduction so that any infection from the wild is not your first infection. The second dose is a second infection so that your body, who now knows what to do, does it and produces lots of the correct antibodies. Your immune system has now had a dry run and is able to react quickly should it ever see those same antigens again.

Clearly, if you have already had that first infection, it may be that you don’t need the first dose. But we can’t really know that so everyone is going need both doses to be confident they are immune. That said, with H1N1 in 2009 it turned out it was a descendant of the 1918 virus which was replaced by one labelled H2N2 in 1957. But the antigens were the same, so if you were born before 1958, you had lasting immunity. That is why 2009 turned out to be not as bad and older people only needed one vaccine dose.

That said, vaccine developers may test the immune reaction to first doses and work out of there is some latent immunity — perhaps from another past coronavirus. So it is possible that, for some people, if we can identify them, only one dose is needed. Suffice it to say, like everything to do with pandemic management, gathering information is the key to generating economic savings. (So apparently I can have insight into things I don’t know much about!)

We have some wriggle room

OK then, this gave me a second question: how necessary is it that the vaccine doses are given exactly 4 weeks apart? Simple logistical challenges tell me that this will be difficult. If we need two doses, some people might want to get the second dose sooner. On the other hand, what if you get a dose and the second one is not easily available? Do you have to start again?

Given what I have already explained, rushing things is a definite ‘no-no.’ You have to be patient and wait for these things. But it is a legitimate question to ask whether moving from 4 to 3 weeks would be a problem. And some vaccine candidates, I understand, are just 3 weeks apart. This might change the attractiveness of certain options.

What about a delay? The good news is that all delay does is delay you getting to confident immunity. You can miss the date for a second dose and still, when you do get it, obtain its full value. To be sure, governments should prioritise giving people that second dose when there are short supplies as doing so produces another immune person then and there. But if there are logistical challenges, we aren’t going to have to do over. That said, I am not sure how long that can be before there is an issue.

Information infrastructure

Regardless, we are going to need good infrastructure to identify who got what dose or what vaccine when. This is especially the case since shortages might cause people to visit different sites for their two doses. One hopes that public health authorities are making these investments right now. They are not technically difficult but you don’t want to be trialling them when we need them to work.


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