Herd immunity is a status that occurs when most of population is protected against or immune to an infectious disease. When a high enough percentage of people in that population are immune to the disease, the small number of people who are not immune are still somewhat protected from the illness because it is unlikely a disease will continue to spread if it has few people to infect.

The percentage of people who need to be immune to a disease in order to protect the whole population is different for every virus. It usually ranges from 50-95%. It also depends on how contagious the disease is, how the virus might evolve and mutate, and how people interact with one another.

The World Health Organization estimates that achieving herd immunity against COVID-19 will require 60-70% of a population to be immune to the virus. However Dr. Anthony Fauci, the United States' top doctor, believes the percentage needed for herd immunity will range from 75-85%, although this is not an exact percentage.

Some scientists do not believe the world can achieve herd immunity against COVID-19 very soon. One of the most obvious challenges the world faces when it comes to reaching herd immunity is vaccine access. Right now, vaccines are largely concentrated in high-income nations. Vaccines are not readily accessible in lower income countries, where a majority of the world's population lives. Until a very high percentage of the global population is vaccinated, we will not achieve herd immunity. Vaccine hesitancy and different immunization levels in different regions further complicate vaccine access and make the goal of herd immunity a difficult one to achieve.

The other challenge that herd immunity is up against is the evolution of viral variants. If the virus naturally mutates enough, some variants may end up not responding to vaccines as strongly. That means individual levels of protection might not be as high as clinical trials demonstrated. Because of viral variants, a vaccine that might have originally prevented a person from becoming infected entirely might eventually only prevent them from having severe symptoms of COVID-19. This could mean that person might have enough of the virus in their bodies to transmit it to others.

We also don't know how long protection from vaccines last. If the virus is still circulating when that unknown period of protection ends, we can expect more cases to occur. While our current vaccines have shown a strong ability to prevent disease and severe symptoms, we do not know exactly how much they can reduce transmission of the virus.

National and international rules for preventing COVID-19 transmission are beginning to change. Some places are allowing residents to travel on planes, eat inside in restaurants, and not wear masks inside. As places begin to open up and these rules are reduced, we will likely see more viral mutations and increased infection rates, because we have not yet reached a level of protection where we can return to pre-pandemic rules without seeing more infections.

COVID-19 may even end up being seasonal virus, tending to spread more readily in winter than in the summer. Rates of herd immunity might need to be higher in January than in July.

Instead of reaching herd immunity by vaccinating the world, we first may need to work to reduce hospitalizations, deaths, and severe infections caused by COVID-19. This may be a more realistic goal for the short-term.

It remains imperative to vaccinate as many people as possible, to protect those vulnerable to infection, and to continue using prevention measures to stop the spread of the virus.

Herd immunity is a frequently cited phenomenon by global health leaders. This lofty goal is one that requires mass vaccination at high percentage across nearly every geography. Because of how few people have access to vaccines, the political context behind this decision is complicated and has caused uproar among those with and without vaccine equity.