Waterloo Science Professor Niayesh Afshordi has been applying tools from observational cosmology to infectious disease modelling, so we asked him to address our questions and tackle some of the persistent misinformation in public discourse about herd immunity. What are the risks associated with it? Why have scientists mostly dismissed the idea? 

What does “herd immunity” mean?

When a virus spreads in a community, every infected person, on average, infects R other people, who in turn may infect more people after some incubation period. The actual value of R varies with the viral strain, season, community, social habits, and other local conditions. Outbreaks can happen when an infected person enters a community where R is bigger than 1.

But what if an infected person enters a community in which a fraction of people has already developed immunity (either through vaccination or prior infections) to the virus? Then, some of the exposures to the virus will not lead to new infections, which effectively reduces the reproduction number, R. If a large enough fraction of the community has immunity such that normal daily social interactions lead to R less than 1, then it is said that the community has reached “herd immunity.”

To contain COVID-19, public health experts say the rate of transmission (R factor) needs to be 1.0 or less. How would herd immunity interact with this goal?

Any level of immunity in the population (through vaccines or natural infections) could work in concordance with non-pharmaceutical interventions such as contact tracing, face masks, social distancing, or stay-at-home orders to reduce the R in the community. For example, communities that have experienced prior outbreaks (assuming that prior infections lead to lasting immunity) may require less strict mitigation measures to reduce R below 1, in order to prevent future outbreaks. Some regions that have experienced severe COVID-19 outbreaks may have already reached the herd immunity threshold, implying that the number of new daily infections should decrease over time, even without non-pharmaceutical interventions. However, there may still be smaller outbreaks due to seasonal variations, or if the immunity fades away over time.

Is there a use for herd immunity as one tool in a broader strategy?

It is important to understand that herd immunity is NOT a strategy, but rather a piece of information that can be used to design a strategy to control the epidemic within a community. In a rural community without many social contacts, it may be feasible to reduce R below 1 using non-pharmaceutical interventions, for a sustained period of time, until a vaccine becomes available and widely administered. 

However, more urban communities require stricter measures to keep R below 1 that may not be sustainable for a long period, for example due to their impact on their local economies. In this case, the community may decide on a combination of restrictions and protective measures to keep the new infections at a low level (and avoid overwhelming the healthcare system), until herd immunity is reached. This is a risky strategy, as it could lead to many deaths, and other long-term conditions, due COVID-19 infections, unless the most vulnerable citizens are well protected from those infected. That is why it is still necessary to use some level of non-pharmaceutical measures (such as face masks, contact tracing, and/or stay-at-home orders) to minimize the daily new infections, and its possible spread to those vulnerable.

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