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Sharp Health News

What is herd immunity?

Nov. 18, 2020

Illustration of people in a health bubble

As COVID-19 made its way across the globe, and countries implemented a variety of preventive measures to slow its spread, one possible solution was considered by both governments and the public: achieving herd immunity to conquer COVID-19.

However, not everyone truly understands how herd immunity is achieved and how many of "the herd" would need to have immunity to COVID-19 to prevent its widespread transmission.

Achieving herd immunity through vaccination
Herd immunity occurs when a large portion of the population becomes immune to an infectious disease, usually through vaccination - limiting the risk of infection passing through a community from person to person. When the majority of people are vaccinated and become immune, those who are not immune benefit from the limited spread of the disease. In essence, those with immunity act as a barrier or buffer around those without immunity, so that the infection is less likely to reach them.

"The control of the spread of measles is an excellent example of herd immunity at work," says Dr. Stephen Munday, medical director of Epidemiology and Provider Safety with Sharp Rees-Stealy Medical Group. "To reach true herd immunity against measles, 19 out of every 20 people must be vaccinated to properly protect the most vulnerable people in the population. However, due to the success of widespread vaccination at eliminating measles, measles vaccination rates dropped, which resulted in an increase in measles outbreaks because people became complacent."

Unfortunately, there is not yet an approved COVID-19 vaccine. And when one does become available, our ability to achieve herd immunity relies on the majority of the population getting vaccinated. In addition, COVID-19 vaccines may require more than one dose and immunity could diminish over time, which means vaccines would need to be repeated on a regular basis, much like the annual flu vaccine.

Achieving herd immunity through infection
Another way to reach herd immunity is through several members of a community naturally becoming infected with a virus. Once infected, the body produces antibodies, which provide a measure of protection from future infection. However, experts estimate that approximately 70% of the population would have to become ill with COVID-19 to reach herd immunity through infection in the U.S.

Along the way, some of those people would likely become severely ill, potentially leading to an overwhelmed health care system, and many could die. What's more, it is unknown how long the antibodies from infection will last, so they may only provide protection against reinfection for a limited time.

A quest for herd immunity gone wrong
Early in the pandemic, experts in Sweden thought they might be able to control the spread of the novel coronavirus without the lockdowns or mask mandates seen in other countries by relying on herd immunity through infection. Counting on citizens to use common sense while going about their everyday routines without limits, they hoped that by May, a large enough percentage of people - 40% in their original estimation - would become ill, build up enough antibodies to acquire immunity, and help protect the others.

Their strategy did not work as anticipated. According to a study released in August, only 15% of the population in Sweden had gotten sick enough to build the antibodies necessary to be immune by May, leaving the remainder of the population without immunity and unable to help prevent the spread of the virus. As a result, Sweden experienced higher rates of viral infection, hospitalization and mortality compared with neighboring countries that implemented strict lockdowns in March.

What to do as we wait for a vaccine
"We have seen the countries that mandated early lockdowns, masking, and used extensive testing and contact tracing experienced greater success in slowing the spread of the virus as compared to countries that were slow to react to the pandemic," Dr. Munday says. "While we look forward to enjoying the benefits of herd immunity in managing the spread of COVID-19, this will not be possible until we have an effective vaccine available for - and taken by - the majority of the population."

Therefore, until a COVID-19 vaccine becomes available and is widely distributed, Dr. Munday and colleagues recommend that people continue to follow COVID-19 prevention guidelines. This includes everyone practicing social distancing and wearing face coverings over their nose and mouth in public settings; avoiding people who are sick; staying home if unwell; and regularly washing hands or using hand sanitizer with at least 60% alcohol. Also, people shouldn't mingle with others not in their immediate household - no social events, parties or gatherings - even during the holidays.

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