It has been nearly eight months of social distancing and mask-wearing since the coronavirus first started spreading rapidly in the US. Initially, many states predicted that a four to six week mandatory quarantine would halt the spread of the virus and allow us to return to normalcy. However, that was far from reality. Initial forecasts predicted that we would get over the pandemic during the summer, but now those forecasts have changed. The supposed “end” to the coronavirus pandemic has been delayed, and delayed, until the one question on almost everyone’s mind is: “When will this all end?” One potential ending is when the population becomes herd immune to the coronavirus.
Herd immunity is a state in which a population becomes immune to a disease, where the risk of the disease continuing to spread is eliminated or greatly reduced due to enough, but not necessarily all, people in a population gaining immunity . The percentage of immune people in the population required for herd immunity is known as the herd immunity threshold . Not every person in a population is necessarily immune, but enough people are immune that they serve as a barrier to disease spread, which greatly lowers the rate of infection. Generally, there are two primary pathways to gaining herd immunity: vaccination or mass infection.
Vaccines are drugs that are administered as a pre-emptive defense against a pathogen. As more people in a population are vaccinated against a disease, they can no longer contract the infection nor spread it to someone else. As a result, vaccines protect both the person who is vaccinated as well as anyone else they might contact. This makes vaccines very effective in achieving herd immunity in a population . However, vaccines are a recent discovery in the scope of human history.
Prior to the discovery of vaccines, herd immunity was achieved through more natural means: mass infection. This process is exactly as it sounds: as a disease spreads throughout a population, individuals will develop a resistance to the disease and be immune after the disease has run its course. However, the caveat is that this pathway could cause widespread illness in the population and potentially death . For this reason, finding a vaccine to rapidly immunize a population is of the utmost priority when confronted with novel diseases.
A key historic example of the importance of vaccination in achieving herd immunity was the spread of polio during the first half of the 20th century. The polio virus initially emerged through sudden simultaneous outbreaks in Europe and the United States during the late 1800s. Epidemics repeated year after year, with cases rapidly rising to nearly 25 cases per 100,000 people from 1945 to 1954 . Even though mass infections keep occurring, these epidemics affect smaller numbers of people, preventing the population from developing widespread immunity. This resulted in an inability to build up herd immunity, allowing polio to reappear and spread illness and death through the affected populations, year after year. The course of the polio virus did not drastically change until the appearance of the inactivated polio vaccine in 1955 and oral polio vaccine in 1961. The number of annual paralytic cases were greater than 10,000 in the years prior to 1955, but the introduction of both vaccines dropped the annual number of cases to less than 100 by 1964 . This highlights another significant danger with mass infection: it is not a foolproof method of obtaining herd immunity.
Unfortunately, we haven’t been able to produce a vaccine yet to rapidly curb the COVID-19 pandemic. However, we have been seeing mass infection occurring throughout the US. Hypothetically, if mass infection continued to occur without a vaccine in the near future, could we still reach herd immunity?
The answer is unclear. Given that we have no vaccine in this scenario, the only way for people to gain immunity is by contracting the coronavirus and surviving until their body develops immunity against it. To determine the percentage of the population who must contract COVID-19 to develop herd immunity, we utilize a very important statistic known as R0. R0, formally known as the “basic reproduction number,” is a measure of the contagiousness of a disease and is more colloquially explained as the number of people a person with coronavirus will infect, on average .
Every contagion has a unique R0 number, and the R0 is then used to determine the herd immunity threshold of that contagion. Estimates of the threshold for COVID-19 vary greatly, from 40 percent to 80 percent. Mathematician Tom Britton from Stockholm University assumed ideal conditions and developed a model that approximated the herd immunity threshold at 43 percent. The US has a population of approximately 330 million, so 142 million people would need to contract coronavirus to achieve herd immunity . If we also factor in the 2.7 percent fatality rate of COVID-19 cases, the side effect of herd immunity would be 3.8 million deaths. In the last 8 months, there have been a total of about 220 thousand deaths in the US. The current number of deaths is not even one-tenth of the total number of deaths to reach herd immunity, and this number doesn’t even include the people who will survive but suffer in other ways :
“Imaging tests…have shown lasting damage to the heart muscle. This may increase the risk of heart failure or other heart complications in the future. [Pneumonia] associated with COVID-19 can cause long-standing damage to the tiny air sacs (alveoli) in the lungs. The resulting scar tissue can lead to long-term breathing problems.” 
The daunting amount of deaths and illness among people who contract the coronavirus is already plenty reason to minimize the spread of COVID-19. However, there’s yet another barrier to herd immunity: the fact that it might be unachievable. Jeffrey Shaman, a professor of health sciences at Columbia University, conducted studies showing that coronavirus can reinfect individuals. “If people can be mildly reinfected, then herd immunity simply would not work” . Allowing people to get infected with the hopes of reaching herd immunity, only to fail ending the spread of COVID-19, would be devastating.
Given all of the dangers of following this path, it should seem obvious why most governments worldwide are stressing the importance of “flattening the curve” and reducing the spread of coronavirus. By not taking these dangers seriously and ignoring public health safety measures, such as wearing masks and keeping our distance, we not only endanger ourselves, but endanger the population as a whole. Until a vaccine can be safely produced and distributed, reaching herd immunity is no short of a miracle.
Edited by Akshay Govindam
Illustrated by Noor Ghanam