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Exploring herd immunity in fighting COVID-19

“If you just let the virus run wild, you’re going to in fact overshoot herd immunity – you’re going to get more people killed, more people infected than what’s required for herd immunity," says University of Waterloo Associate Professor Niayesh Afshordi.
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With potential vaccines to COVID-19 visible on the horizon, there’s no doubt that many are eager to return to the ‘before times’ after months of navigating through an ongoing pandemic. One dangerous strategy suggested by some toward the beginning of the first wave was that of misinterpreted herd immunity –allowing the virus to spread in order to immunize the population and avoid widespread disruption.

That response, said University of Waterloo Associate Professor Niayesh Afshordi, would likely mean overshooting herd immunity and ultimately, unnecessary infection and death. Specializing in Physics and Astronomy, the professor has been applying observational cosmology to infectious disease modelling – hosting a recent Q&A with University staff to combat misinformation surrounding the scientific process. Speaking with 570 NEWS, Afshordi made his definition of herd immunity clear – specifying its value as a process and eventual goal rather than a lone strategy for fighting the pandemic.

“If there are enough people in a community who are immune to a contagion (…) if it’s an infectious disease but it cannot actually find a person who is susceptible, then it’s not going to spread…” said Afshordi. “For that, you usually need a significant fraction of the population to become immune – either through vaccination or natural infection…”

Afshordi compared the beginning of the pandemic to the experience of rolling a bowling ball downhill – as he said that if you just let the ball continue rolling without intervention, it’s hard to predict where it will end up, eventually overshooting its target. He said that mitigation factors, like masking, physical distancing and contact tracing act as brakes for that rolling figure, allowing us to slow infection rates while we work toward a viable vaccine.

“If you just let the virus run wild, you’re going to in fact overshoot herd immunity – you’re going to get more people killed, more people infected than what’s required for herd immunity.”

The associate professor said that the best way to achieve herd immunity is through vaccination – if 60-70% of the population can be vaccinated, the others may not have enough contacts to be infected. He said if this measure is not possible, mitigation can slow down propagation eventually leading to the goal of herd immunity anyway. Afshordi mentioned that, while a vaccine may be coming, it will more than likely take some time to distribute and administer – adding that the best way to minimize harm due to COVID-19 will likely be in looking at vulnerabilities in different communities.

According to Afshordi, if immunity has started to build through natural infection in one area, that area may be less vulnerable than a location that has not seen any exposure. While public health measures reduce exposure to other potential sources of transmission, those sources being potentially immunized can further assist in keeping virus transmission rates low. He said that it’s important to understand herd immunity not as a strategy, but as a piece of information or a tool that can be used to design a comprehensive strategy to control the epidemic.

Afshordi said that statistics and data on COVID-19 immunization through vaccination and long-term effects of natural infection are still somewhat poor – based largely on anecdotal observation. As that understanding continues to develop, he said health units can continue to optimize their strategy to handle the virus.

“As a disease spreads, immunity does build up – but letting it go is the worst thing you can do.” Said Afshordi. “You still have to use mitigation strategies as much as possible to minimize harm to vulnerable people.”

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