As the effects of the COVID-19 pandemic improve in many places, the question of herd immunity has become relevant once again. Herd immunity is reached when a sufficiently large portion of a population is immune to a contagious disease that the disease cannot effectively spread, so infection rates begin to decrease (whereas individual immunity can be achieved through exposure to the disease or vaccination). Herd immunity is important not only because it could bring an end to the pandemic but because it protects individuals such as babies and the immunocompromised who cannot safely become immune to the disease through exposure or vaccination.
The percent of individuals in a population who must be immune to achieve herd immunity is different for every disease. This threshold is determined by the reproduction number, or R0, of the disease, which is a measure of how many non-immune people are likely to be infected by one contagious person. According to WebMD, the R0 of Sars-CoV-2, the virus that causes COVID-19, is thought to be 2–3, meaning one infected person is likely to infect two to three others who are not immune. This means that 50–67% of a population must be immune before the effects of herd immunity are visible. However, because vaccines do not provide perfect immunity, the Center of Disease Control and Prevention (CDC) estimates that 70–80% of a population must be vaccinated against COVID-19 before herd immunity is achieved.
In some places, the vaccine rollout is already beginning to show its effects. For example, in Massachusetts, 63% of residents have received at least one dose of a COVID-19 vaccine, with 47% of the population fully vaccinated, according to the CDC. Massachusetts saw a spike in COVID-19 cases at the beginning of April, but this spike of the pandemic did not reach the levels of previous waves—the highest average number of deaths per day was 35, compared to roughly 170 deaths per day during the first wave in April 2020 and 80 deaths per day in January 2021, according to CDC data. These numbers suggest that immunity is increasing, and as vaccines have recently been made available to all residents 12 and older, immunity will likely continue to increase. Concord Academy has taken advantage of the resulting decreased risk of transmission by relaxing its outdoor masking protocols.
However, it is still unsure whether all communities, large and small, will be able to reach herd immunity. Vaccine hesitancy, lack of availability, potential concerns about protection from variants, and issues with distribution are four significant obstacles to reaching worldwide herd immunity, and researchers are divided on whether or not these issues can be surmounted. Additionally, maintaining herd immunity has a challenge. For two-dose shots, two appropriately spaced doses are required for maximum efficacy, and it is likely that COVID-19 immunity will require regular booster shots to be retained. Massachusetts may be able to achieve herd immunity, but it will require mutual participation. The best thing anyone can do to help their community reach herd immunity is to take the COVID-19 vaccine available to them and continue to practice risk mitigation protocols for as long as necessary.