Herd Is The Word

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Now that there is a vaccine for COVID-19 on the way, the concept of herd immunity is being talked about more often. The experts are assuring us that this pandemic will finally be over once we have vaccinated enough people to achieve herd immunity. Today I would like to look into some aspects of the concept of herd immunity. Herd immunity occurs when a significant portion of a population becomes immune to an infectious disease, thereby limiting further disease spread. Those who are not immune are indirectly protected because ongoing disease spread is very small. A person can become immune to a disease either by catching it and recovering or by immunization.

The proportion of a population who must be immune to achieve herd immunity varies by disease. For example, a disease that is highly contagious such as measles, requires more than 95% of the population to be immune to stop sustained disease spread and achieve herd immunity. If you recall, we saw examples of this in 2019, before the current pandemic, in outbreaks of measles in different parts of the country. Those outbreaks occurred because of two factors – vaccination refusals by antivaxxers and an influx of sick unvaccinated individuals from outside of the U.S. In addition, measles is much more virulent than COVID as nearly everyone who contracts it will have symptoms, and the death rate, especially among children, is much higher. Getting back to COVID, the actual proportion of immune people required to achieve herd immunity is still unknown, but I have seen numbers around 70%. It is not 95% because coronavirus is not nearly as contagious as measles.

Herd immunity protects those who cannot be vaccinated, such as newborns and those with compromised immune systems. Immunity from either vaccination or disease usually wanes over time, requiring revaccination. It is not yet known how long immunity to COVID lasts, either from vaccination or disease. In some situations, even if a large proportion of adults have developed immunity after infection, the disease may still circulate among children. Herd immunity, even when very high, does not fully protect those who are not immune. Herd immunity reduces the risk of infection but does not eliminate it.

It is not yet known if having COVID confers immunity to future reinfection, in fact there have been a few anecdotal reports of individuals contracting it a second time. Most likely infection does impart immunity, but for how long is unknown. To achieve herd immunity by infection alone would probably overwhelm our healthcare system and cause even more deaths and complications, so the release of a vaccine is most welcome. Even with widespread immunizations starting early next year, it will take time to achieve herd immunity. I have seen estimates of anywhere from five to nine months, which means mask wearing, social distancing, etc. will be with us for quite a while, and normality may not return until mid to late 2021. We can take some comfort, however, in the fact that the death rate from COVID-19 is well below 1% plus only about 20% of those who test positive for it get symptoms.

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By Peter Galvin, MD

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