What does vaccination mean for rapid screening?

The case to screen actually goes up as we roll out vaccinations

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. (I am also part of the CDL Rapid Screening Consortium. The views expressed here are my own and should not be taken as representing organisations I work for.)

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The question I get the most is “given that we are now vaccinating people, is there any need to engage in wide-scale rapid screening for Covid-19?” The answer is yes and, indeed, that the difference between the social and private returns for rapid screening goes up considerably as we vaccinate.

Without vaccines, a rapid screening system breaks chains of transmission and keeps outbreaks to a minimum. Having more vaccinated people does that too. From an economic point of view, in terms of function, rapid screening and vaccines are substitutes in preventing outbreaks.

Given this, let me do two things into today’s post. First, I’ll work through how substitutable vaccines and screens are. It turns out that is subtle. Then I’ll turn to explain why that masks an information gap that needs to be closed.

As you vaccinate, how much less screening can you get away with?

Thanks to Carl Bergstrom and his co-authors we actually have a sense of this. They have developed a calculator that lets you work out how much less screening you can get away with as your population is vaccinated.

The way to look at this is as a workplace. You have a certain number of people vaccinated in your workplace and you want to keep the probability of an outbreak (where maybe 10% are infected causing you to shut down) as low as possible. Even with screening, outbreaks cannot be ruled out. This is all a risk exercise.

Now the issue we have now and going forward is that the basic reproduction number of Covid-19 is now 5 which means that with some interventions you can probably get it down to 3 or so. Of course, in the future, you want none of those interventions either.

Given all this, here are the results:

Here are some things to notice:

• The more people are vaccinated, the lower is the probability of an outbreak. And once that gets above 50% vaccinated, the probability is pretty much zero.

• As you increase the share vaccinated, the marginal cost of moving from semi-weekly to screening every two weeks goes from a 150% reducing in the risk of outbreaks to over 400%. This is because when more people are vaccinated, more intensive testing can have a larger relative impact on the probability of outbreaks. We see the same type of effect in terms of the number of people infected.

Note, however, that while vaccination reduces the chance of having a shutdown level outbreak, at the same time, using more intensive screening to manage that risk becomes more valuable. This is actually intuitive. When there is a big chance of an outbreak regardless of what you do, screening helps but only so much. When that chance falls, screening has more room to protect your workplace.

The bottom line is: while a minority of your people are vaccinated, the value of rapid screening in keeping things open, goes up. After a point, enough people are vaccinated that screening is no longer required.

The problem of waning immunity

Here is where I do my usual thing and depress you further. I stress that this is coming from epidemiologists not economists but that doesn't make it any less disturbing.

I am told by my epidemiologist friends that virtually every other vaccine wears off in older people. That’s why we get shingles when we are over 50. Any reasonable expectation for the Covid vaccines should presume the same thing. If flu is anything to go by, that waning immunity might arise within a year.

Suppose we go along merrily thinking we are now invincible forever once vaccinated. Then people will go back to normal. What’s more, there will still be some coronavirus circulating but it will be background noise. That’s important. It won’t be, “oh there are now 15 cases, we should do something” to trigger us.

We have to combine that with another effect: we vaccinated everyone at more or less the same time. That means the immunity doesn’t wane smoothly but instead on the same schedule for certain groups — with those we vaccinated first being first to see it. And let’s add to that the likelihood that if there is an increase in illness amongst the elderly that might not look like Covid did before.

What this adds up to is that Covid could re-emerge and do so very quickly. You may have thought we waited too long last time to act. In this scenario, the reaction time may be way longer. That means we could find ourselves very quickly overwhelmed.

This raises a big information problem: how will we know? When most people are vaccinated, there will be less screening and less vigilance. Add to that, a natural form of denial and it is unclear how we tell whether this is turning into an imminent problem or not. What is worse, this is not a matter of gathering information for privately profitable reasons — that is fairly low then unlike today. Instead, we need the information for social reasons. The value of information for that purpose has gone way up.

In terms of the earlier discussion of reducing screening when a majority are vaccinated, waning immunity causes that figure to creep downwards. Thus, if we think we are done with screening, we leave ourselves without the ability to monitor the situation so we can see sooner whether waning immunity is a problem and act on it.

The solution

We need a plan for surveillance of the population — the vaccinated population — for changes in the level of immunity. We need to know who was vaccinated when and we need to conduct screening on that basis. This is very obvious. But my concern is that we haven’t been gathering key information regarding who was vaccinated when to conduct a systematic public health surveillance initiative starting in the summer. This information gap needs to be closed and we need to build the infrastructure to do that ASAP.