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Three Questions

Three Questions about COVID-19 Infection and Immunity

We checked in with Yale SOM’s Dr. Howard Forman about herd immunity, vaccines, and that case of reinfection in Hong Kong.

Travelers at San Francisco Airport on August 11, 2020. Photo: David Paul Morris/Bloomberg via Getty Images.
Travelers at San Francisco Airport on August 11, 2020. Photo: David Paul Morris/Bloomberg via Getty Images.
“While there may be some very small areas that have achieved herd immunity, no large region, and certainly no state, is even remotely close to this.”

Does the decrease in new cases from the summer peak mean that Americans are reaching herd immunity? Is that a desirable outcome?

The decrease in cases since the outbreaks of June and July appear to be due to rigorous attention to social distancing, masking, testing, isolation, contact tracing, and other ordinances and measures taken to reduce spread (not to mention fear of infection). These efforts are not perfect, but they seem to have reduced the spread of the virus in most areas.

There is no evidence of herd immunity. Obviously, in areas that have more previously infected individuals, there are fewer susceptible and that helps to reduce the height of future outbreaks. But the definition of herd immunity is quite specific: it is a level of immunity in the community that makes further outbreaks impossible. While there may be some very small areas (including, perhaps some prisons at a single point in time) that have achieved herd immunity, no large region, and certainly no state, is even remotely close to this. In order to achieve herd immunity through infection on this scale, we would need to accept tremendous morbidity and mortality; assume that we know enough about long-term harms of SARS-CoV2 infection; and assume that infection results in durable immunity—no new infection or new disease. I think it is a bad premise and a dangerous experiment to try.

How should we interpret the recent case of a 33-year-old man in Hong Kong who contracted COVID-19 for a second time? Does it mean that a vaccine won’t be effective?

The case from Hong Kong University is a first and we need to see more evidence before we assume it is representative. But it does appear that an individual was truly re-infected with the SARS-CoV2 virus, just a few months after an initial infection. The good news is that he did not have symptoms during this new infection. The bad implication is that he was seemingly capable of spreading this infection to others, thus making herd immunity not just unlikely but impossible. The entire notion of herd immunity, as mentioned above, is based on the idea that once you have been infected and have immunity, you can not be a source of new spread.

This case does not tell us enough about the effectiveness of a vaccine. We would certainly like to see a vaccine that not only protects against disease but infection itself. And we obviously need it to be durable (not just for a few weeks or months). Only time will tell how this will work out.

How far are we from having a vaccine and distributing it widely enough to make a difference?

We could have a vaccine today if we were to accept the risks and unproved efficacy of untested products. We have vaccines in various stages of testing, and manufacturing is not the limiting step. We hopefully will have a widely available, effective, safe vaccine, or more than one, in the early part of next year, and I would hope that they are broadly distributed enough to protect the herd ahead of the 2021 fall semester.

Department: Three Questions