COVID Transmissions for 5-4-2021
Herd immunity?
Greetings from an undisclosed location in my apartment. Welcome to COVID Transmissions.
It has been 533 days since the first documented human case of COVID-19. In 533, Emperor Justinian I appointed the general Belisarius to reconquer much of the Roman Empire’s lost territory—which he did!
Speaking of comebacks, I’m hoping we don’t let COVID-19 make one. To prevent that, we need to keep the vaccinations coming.
Today I’ll discuss an expansion of the COVID-19 vaccine to younger children in the US, and also we’ll talk through the concept of herd immunity.
As usual, bolded terms are linked to the running newsletter glossary.
Keep COVID Transmissions growing by sharing it! Share the newsletter, not the virus. I love talking about science and explaining important concepts in human health, but I rely on all of you to grow the audience for this, which you can do by using this button here:
Now, let’s talk COVID.
US FDA poised to authorize Pfizer vaccine for 12-15 year old children
Various outlets are reporting that some time in the next week, the US FDA will be authorizing the use of the Pfizer mRNA vaccine in children ages 12 to 15. Here’s the New York Times story on the matter: https://www.nytimes.com/2021/05/03/us/politics/coronavirus-vaccine-teenagers.html
This would be fantastic news. I think that the data support it, and the more people that are vaccinated, the better.
Understanding “herd immunity”
Another New York Times story that ran today offered the bleak headline that “herd immunity” won’t be happening in the US, and made quite a splash in that process: https://www.nytimes.com/2021/05/03/health/covid-herd-immunity-vaccine.html
I have some issues with this article. I think it has entirely accurate information, but I think it presents it in a confusing way that makes vaccination seem futile, which is very much not the case. Much of what I’m about to say is actually in the article—I’m just providing a little more explanation that I think will help us manage our expectations for what vaccines can do.
First, I want to discuss the concept of herd immunity, which I think is not explained well in the article. Herd immunity, strictly defined, is a situation where so many individuals in a population are immune to an infectious disease that even if one isolated person with the disease enters the community, any remaining susceptible people are insulated from the infection by a cordon of protected individuals. Under such conditions, any outbreak that occurs quickly extinguishes itself.
The threshold for protection above which this effect is observed is called the Herd Immunity Threshold.
But I think we need to be realistic about this effect. That threshold just refers to a situation where so many people are immune to the disease that it extinguishes itself very rapidly. This is an important threshold because it facilitates things like disease eradication. This doesn’t mean that herd immunity effects are absent below this threshold, they just aren’t enough to completely contain an epidemic on their own.
Every vaccine protects the individuals who get it to some extent. We know that. This also means that these individuals can disrupt transmission chains and protect others from COVID-19. This is particularly true if a susceptible person is surrounded entirely by vaccinated people. Imagine, for instance, that someone’s homebound grandmother is not yet vaccinated, but only interacts with three other people. If all three of those people are vaccinated, herd immunity benefits will protect the susceptible person in this equation, regardless of how many people in the wider community are vaccinated.
Yes, it would still be better, especially considering the possibility of breakthrough infections, for more people to be vaccinated in the wider community, but there is still some meaningful protection being offered here to the unvaccinated person.
If we expand this to a wider set of people, whole communities can be protected in this fashion even if the wider world they exist within have not reached the meaningful threshold. Vaccination is not evenly distributed, with some regions having better uptake and others worse uptake. While I would want it to be 100% everywhere, a local community with a good vaccination rate is going to see some good effects.
But let’s imagine that there is an even distribution of vaccination. Even in this case, sub-threshold vaccinations will still provide something like a herd effect. Because fewer people in the population will be susceptible, it will take more interactions between an infected person and a random other person in the community for a new infection to be produced. This is because there will be a higher chance that each random interaction will be with a protected person. That effect is present at any level of vaccine uptake, but obviously the more vaccinated people there are, the better. It may not be enough to make new outbreaks disappear quickly, but it can slow them down. Slowing them down means that localities can react to new outbreaks and can try to isolate the affected individuals to reduce the impact. That is a huge benefit!
Remember that vaccination is not our only disease-control measure. We still have masking, contact tracing, and isolation of infected cases. These measures have meaningful impacts that can contain outbreaks, and when used in combination with highly effective vaccines, they are even better.
One thing that is especially problematic about COVID-19 is that it spreads quite effectively when no disease control measures are taken, with multiple new cases being created, on average, for every individual case detected. Each vaccination in a population can help reduce the number of new cases—the “effective reproduction coefficient”—that are created from a new case, because like I mentioned earlier, the more protected people there are, the less chance of spread there is. The lower that the reproduction coefficient gets, the better. Generally, if you reduce it below 1, an outbreak will disappear gradually. If it is above 1, it will grow, but again, the closer it gets to 1, the slower the outbreak grows.
So, even if a population has a vaccination rate below the “herd immunity threshold,” there are real population effects of the vaccination rate.
We have seen some of these effects in places like Israel. There, vaccine uptake is around 60%, but cases of COVID-19 have dropped dramatically. This is because Israel had a vaccination campaign combined with other disease control measures for long enough to slow and then reverse the spread of disease in the country. We can also do that here.
There is also the possibility that the herd immunity threshold, or at least the number of vaccinated people needed to reach it, is much lower than the 85% that people have been assuming. That possibility, mentioning the Israeli data, is discussed in this Vox article: https://www.vox.com/22400322/vaccines-herd-immunity-coronavirus-israel
One way or another, we can see from the Israeli experience that lower vaccination rates than 85% can still have meaningful effects in reversing COVID-19 surges. The theoretical herd immunity threshold is not the be-all and end-all of the equation here. There are real population effects of a high rate of vaccination, even if it isn’t “high enough.”
Then there’s the final, and arguably most important, benefit of vaccination: it protects the individual who gets vaccinated! And it does it safely, with a very low rate of serious adverse events.
While I think that the NYT article that I linked at the top is correct, and we may never reach a vaccination rate of 85%. But I think that that number is only important if eradication of COVID-19 is the goal, and I don’t think that that goal is realistic. There was a time—early on in the history of this newsletter—that I thought eradication might be possible, but I no longer think this way. COVID-19 is too widespread in the world, and there are too many other animals that can play host to SARS-CoV-2, for us to realistically expect to eradicate the virus. Only two viruses have ever been eradicated, and in the case of the one human virus, smallpox virus, it took almost 200 years to accomplish it. Smallpox was easier, too, because it only infected humans. That isn’t the case with SARS-CoV-2, which has been shown to infect everything from house cats to zoo gorillas.
If eradication is off the table, I think this high herd immunity threshold matters a whole lot less. We’re not aiming to eradicate SARS-CoV-2, we’re looking to reduce the levels of COVID-19, particularly severe COVID-19, to where most people no longer have to worry about the disease. I would be pretty OK with a future where so many people are protected that the worst most people ever hear about COVID-19 is that it gave someone they know a brief, cold-like illness. That is possible, as we see from the Israeli experience, with much less than an 85% vaccination rate. To me, that represents a good level of population immunity. And I think, even if SARS-CoV-2 is not completely eradicated, we can get to this kind of relationship with COVID-19, the disease that it causes.
What am I doing to cope with the pandemic? This:
Job interview
Yesterday I had an interview for an internal position at my current company that would be a substantial promotion. I hope it works out!
You might have some questions or comments! Send them in. As several folks have figured out, you can also email me if you have a comment that you don’t want to share with the whole group.
Join the conversation, and what you say will impact what I talk about in the next issue.
Also, let me know any other thoughts you might have about the newsletter. I’d like to make sure you’re getting what you want out of this.
Part of science is identifying and correcting errors. If you find a mistake, please tell me about it.
Though I can’t correct the emailed version after it has been sent, I do update the online post of the newsletter every time a mistake is brought to my attention.
No corrections since last issue.
See you all next time.
Always,
JS
I've been disheartened to see this recent wave of COVID defeatism arising out of the apparent difficulty of achieving "true" herd immunity. The point isn't herd immunity, the point is preventing death, suffering, and massive social disruption.
That said, I think that, in addition to doubling down on vaccination -- perhaps including carrot of cash incentives, the stick of school and employer mandates, or both -- we badly need: 1.) more, better, and cheaper testing and 2.) therapeutics (steroids and antivirals seem most promising here).
We also urgently need to understand long COVID and what can be done to prevent and manage it. The NIH is providing $1.15 billion over four years to study the issue, and this is certainly a step forward. But we can't continue flying blind for half a decade. Are we seriously contemplating sending people back to school and work knowing full well that COVID infection is still a serious risk, and that as many as a third of those who are infected, regardless of initial symptoms, will develop serious chronic conditions? How is that anything other than morally reprehensible?
I'm also a little concerned with the (seemingly culture-war-driven) geographic disparity of vaccination, which could leave us with islands of relative immunity surrounded by COVID-ravaged hinterlands.