We’re Not In the Herd Yet…
Three Paths to COVID-19 Herd Immunity
As we discussed in an earlier blog, Herd Immunity, in which so many people have antibodies that the virus can no longer spread, is how we can return to a normal life. That blog offered three paths to herd immunity:
- “Let ‘Er Rip!” A widely criticized proposal essentially says, find some way to isolate the most vulnerable people and let everyone else catch coronavirus, so that most of the population acquires protective antibodies. Unfortunately, you can’t effectively protect just a few people, and this approach would lead to 1.3 to 1.5 million additional US deaths.
- Broad Vaccination. Another way to conquer COVID-19 herd immunity delays is simply to vaccinate a lot of people, quickly. As of February 17, 2021, US people received 55 million doses, reaching 11.9% of the population with at least one dose, and 4.5% with two doses. The current rate of vaccination is 1.8 million shots per day. This needs to accelerate if we are to vaccinate all of the willing by late summer.
- Broad, Weak Infections. Our previous blog presented evidence that widespread wearing of face masks could weaken the severity of coronavirus infections. That would allow many people to acquire weak, nonfatal infections. Such infections would make them resistant to further disease, thus bringing society closer to herd immunity.
COVID-19 Herd Immunity Delays
As previously discussed, herd immunity is a complicated topic. If we had a homogeneous society, in which every person randomly encountered every other person, the virus would mostly die out once we reached a certain percentage of resistant residents. But of course, human interactions are nonuniform, “lumpy” and variable. We might attain herd immunity in one town, only to have its immunity punctured by the first visitor who arrived from somewhere else.
Despite these limitations, we can’t avoid talking about herd immunity as a single percentage goal, because it’s the only measure we have. We are far from having validated fine-grain models of how people interact, and we don’t have a set of accepted measurements to describe group immunity.
OK, if a single number is all we can point to as a goal, what is that number? People have estimated herd immunity levels everywhere between 10% and 75%. And I’m sorry to report to you that we now have an instance where an immunity rate of 76% was not sufficient to stop the coronavirus!
76% Infected Did Not Stop the Virus
Manaus is the capital and the largest city in the state of Amazonas, Brazil. It has 2.2 million people. The city covers an area about the size of Connecticut, with one-half of Connecticut’s population.
Manaus was the site of a mostly uncontrolled outbreak of coronavirus during most of 2020. A group of 37 researchers from Brazil, US and UK analyzed the data and published their findings in Science magazine. The same issue contains an editor’s summary which will be easier reading for those who wish to consult it.
The researchers concluded that 66% of the people in Manaus had COVID-19 infections by June 2020, and the percentage rose to 76% by October. However, even this high rate of infection did not attain herd immunity.
Implications of the Manaus Study
What can we learn from the study of coronavirus in Manaus?
- Mild infection, brief protection: One piece of good news is that many of the infections were mild, probably because Manaus has a relatively young population. However, the resulting antibodies from these infections declined in only a few months. Therefore, recovered patients might not have good protection for very long.
- Don’t rely on natural immunity: The study found that most people who have not encountered coronavirus are susceptible to it. Therefore, we can’t count on natural immunity as a source of protection, at least not in populations similar to that of Manaus.
- Don’t rely on shielding the elderly: Infections appeared to spread uniformly across the population and were not limited to people having more frequent exposure. That suggests that it is not practical to shield just the most vulnerable people. Inevitably, if the virus is circulating in the community, it will find its way to the people who are most at risk of dying from it.
To this we should add a concern: Because the coronavirus is constantly mutating, even if we attain herd immunity for one strain, we may still be at risk when a new variant appears.
The Way Forward
A single study of a single place is just that. It is suggestive but far from conclusive. However, as the researchers correctly state:
Manaus represents a “sentinel” population, giving us a data-based indication of what may happen if SARS-CoV-2 is allowed to spread largely unmitigated.
At a minimum, Manaus suggests that defeating coronavirus will require more than a single “silver bullet,” whether it be vaccines, isolation of infected people or free-ranging infections. We need a multi-pronged approach with most or all of the following:
- Non-pharmaceutical approaches, that is, social distancing, face masks, clean air and hygiene.
- Reliable systems to test, trace and isolate infected persons.
- Travel restrictions such as testing or quarantine, as is already required by Alaska and Hawaii, and many countries.
- Improved treatments for the disease.
- Accelerated distribution and administration of vaccines.