Throughout the pandemic, I’ve heard the term herd immunity bounced around. What exactly is that?
Herd immunity occurs when enough individuals in a community have acquired immunity against a particular infection to make person-to-person transmission less likely. An individual’s immunity can be acquired by either getting vaccinated against the infection or contracting the infection. Herd immunity is a great example of safety in numbers.
We primarily need herd immunity to prevent or at least slow the spread of an infectious disease. The greater the percentage of people in a community who are immune, the lower the chance that pathogens will be passed to uninfected people who aren’t immune. This protection is particularly important for people who can’t be safely vaccinated or whose immune systems won’t adequately respond to vaccination.
One challenge of herd immunity is that immunity from vaccination or previous infection can wane over time or because the virus changes. In some cases, people may need to be revaccinated regularly, as is done with annual flu shots. With Covid-19, immunity from the first two shots of the mRNA vaccines made by Moderna and Pfizer wanes after several months and a third (booster) shot is now recommended for everyone age 12 and older. To date, we do not know whether further immunization will be needed, though that may be the case. Also, the Omicron variant has evolved to partially evade the immunity we get from both vaccination and previous infection.
This points to another challenge, which is that herd immunity relies on the individual actions of citizens. A significant percentage of people don’t receive the recommended vaccinations because of the belief that vaccines are harmful (though getting Covid is more harmful than getting the shot), lack of access, or cost (though Covid vaccines are free and now readily available). When enough people living in proximity to each other don’t get vaccinated, it lowers the level of herd immunity and allows disease outbreaks, such as with clusters of measles in recent years and the ongoing coronavirus pandemic.
