You’ve probably heard of herd immunity—but do you know what it actually means? In this blog post, we explore what herd immunity is, how it realistically can be achieved, and epidemiological insights into herd immunity and COVID-19. 


According to the Centers for Disease Control and Prevention, herd immunity is defined as, “A situation in which a sufficient proportion of the population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely.”


Since such a large number of the population needs to develop immunity from a given virus in order to reach herd immunity, it’s extremely difficult to achieve herd immunity based on natural infection and recovery alone. That’s why the best way to achieve herd immunity is through vaccinating a large percentage of any given population against an infectious illness. The amount of the population that needs to be vaccinated for herd immunity to be possible can depend on the virus itself and how infectious it is. However, typically 50%-90% of the population must be immune to said disease in order to achieve herd immunity, according to Johns Hopkins Bloomberg School of Public Health. 


There’s good news—herd immunity absolutely is possible and has been achieved throughout history! One famous example is that of polio. Due to the invention of the polio vaccine in the 1950s by Dr. Jonas Salk (and a second polio vaccine later developed by Dr. Albert Sabin), as well as widespread vaccination efforts globally, polio was successfully eradicated in the U.S. in 1979. Globally, polio has nearly been eradicated, with all but two countries (Pakistan and Afghanistan) having completely stopped polio in its tracks. According to the World Health Organization, polio cases worldwide have decreased by 99% since 1988, from an estimated 350,000 + cases down to 22 reported cases in 2017. 


Experts estimate that in order for herd immunity against COVID-19 to be achieved, it would require around 80% to 90% of the population to have COVID-19 immunity, either through prior infection or through vaccination. However, we still don’t know for sure how long prior infection can provide immunity against reinfection of COVID-19, though some studies suggest protection lasts around six months. This is why getting the COVID-19 vaccine—rather than getting infected with COVID-19 and hoping you recover—is a much safer bet. 

“Vaccines are crucial for giving us a chance in reaching herd immunity, particularly for a pathogen like SARS-CoV-2,” says STChealth’s Chief Epidemiologist, Dr. Kyle Freese, PhD, MPH. “If we allow a pathogen to spread through a community unchecked, in addition to the obvious adverse effects on that population—such as unnecessary morbidity and mortality—we are giving the virus more opportunities to replicate and thus mutate. If a mutation is achieved that proves resistant to whatever natural immunity is gained from a previous strain, we essentially have a fresh, susceptible population and the cycle can repeat itself.”


While recovery from natural infection can result in immunity in individuals for certain viruses, it is highly unlikely that this will happen at a fast enough rate to result in herd immunity and for said viruses not to continue to mutate. Furthermore, the potential risk of death is too high of a toll for us to pay as a society. 

Take Sweden’s approach to handling the COVID-19 pandemic, for example. Rather than implementing preventative measures against COVID-19 such as lockdowns, social distancing, and requiring masks, Sweden tried an opposite approach (prior to COVID-19 vaccines becoming available)—largely remaining open for business with little COVID-19 restrictions, in the hopes of obtaining herd immunity. It didn’t work. 

According to an article published by Reuters, herd immunity was never obtained in Sweden with this approach. In fact, according to the same article, the highest daily average of reported COVID-19 infections in Sweden was at its peak on November 12, 2020, with an average of 4,625 new infections reported each day. 

The report goes on to say, “At the time of this article’s publication, there have been 257,934 infections and 6,891 confirmed coronavirus-related deaths reported in the country since the pandemic began…According to mortality analyses from the Johns Hopkins Coronavirus Resource Center, the case fatality rate in Sweden is 2.6%—higher than that of neighboring Finland (1.6%), Norway (0.9%) and Denmark (1.0%), as well as the United States (2.0%). As a country, Sweden has had 66.76 COVID-19 deaths per 100,000 people, compared to 7.23 in Finland, 6.28 in Norway, 14.59 in Denmark, and 82.72 in the United States.”

Time and time again, it has been proven that the best path towards herd immunity is disease prevention and vaccination. If you haven’t yet, we urge you to get the COVID-19 vaccine. For more resources about the COVID-19 vaccine, the CDC is a great place to start. And, for COVID-19 immunization records and other immunization records, be sure to check out

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Herd Immunity Chain