Recent published studies from England, Spain, and Sweden suggest the infeasibility of herd immunity to Covid-19 in the short term. The rapid drop in antibody levels following infection demonstrated in these studies indicates that coronavirus immunity may not be long-lasting, and that the concept of herd immunity by itself is not achievable without additional support in the form of regular vaccinations or additional treatments to improve immune response.
The argument that natural herd immunity will resolve the coronavirus pandemic relies on the assumptions that infection rates are high and that antibodies confer long-term immunity. European studies suggest the opposite. Infection rates are lower than expected where the virus is thought to be endemic, and antibodies decline rapidly, suggesting that natural herd immunity is an unlikely outcome.
In an interview with the author, Todd Ellerin, Director of Infectious Diseases at South Shore Health, stated that “the flattening of the curve is overwhelmingly from behavioral actions and not herd immunity. Physical distancing and mask wearing are the two most important things that have flattened the curve and, to some degree, but not nearly where we need to be, the testing, isolation, and tracing. Expert epidemiologists say that roughly 7% of the U.S. has been infected. It is not clear that we can even get herd immunity to Covid-19. With Influenza [seasonal flu], we don’t achieve herd immunity; tens of millions of people still get it. The data is making us concerned that this is going to fall in line with the other common coronaviruses, where generally we think you have immunity for maybe six to twelve months, but then you lose it and can get reinfected. That’s still a hypothesis, and we need time to understand if we are going to be seeing more reinfections. The key to this is some people estimate that this will be around for up to five years, and the seasonality is unclear.”
A study from King’s College found that Covid antibodies decrease at a constant and rapid rate, regardless of infection intensity; this suggests that, even if a sufficient antibody level is achieved through vaccination, the effectiveness will diminish over time. The study found that the level of antibody response is correlated to the severity of infection, with antibody levels peaking after an average of 23 days regardless of severity. Most significantly, the mild or asymptomatic cases had no detectable antibodies after two months. These findings imply that, even if an effective Covid-19 vaccine is produced, the vaccine may have to be administered yearly or at a yet undetermined interval to maintain immunity.
Recent data from Sweden and Spain support these statements. In Spain, approximately 5% of the population tested positive for the virus in a study of 60,000 randomly selected individuals, despite Spain being a Covid hotspot. In Sweden, where a strategy of herd immunity through community spread is being attempted, only 6.3% of individuals tested positive for coronavirus.
In a recent opinion piece in The New York Times, Dr. Akiko Iwasaki and Dr. Ruslan Medzhitov, both professors of immunobiology at Yale, argue that dropping levels of Covid antibodies after acute infection may not indicate a decreasing level of immunity. These doctors suggest that focusing primarily on antibody levels, as these studies do, is misguided.
Focusing instead on T cells and B cells, which provide long-term protection, these doctors point out that natural infection with some viruses does not provide the same level of robust immunity as does a vaccine. This suggests that, even if coronavirus infection does not confer sufficient long-term immunity, a vaccine may be able to. Thus, Drs. Iwasaki and Medzhitov argue that, while natural herd immunity may not itself be achievable, a vaccine may lead to widespread long-term immunity.
Hemanth Nair, a former CDC Epidemic Intelligence Officer and a former Epidemiologist at the New York Department of Health and Mental Hygiene, said the following in an interview with the author:
“They’ve done studies with the Diamond Princess population, a closed population because they were all on a cruise ship, and the death rate there was around 1%. Without a vaccine, if we all just went out, without any type of non-pharmaceutical intervention, we could expect at least that proportion of people dying from coronavirus.”
The mortality rate has not been reduced in the age group sampled in the Diamond Princess study. Currently, the best CDC estimate of the infection mortality rate, which includes all age groups and asymptomatic cases, is 0.65%.
According to the Mayo Clinic and Johns Hopkins University, herd immunity from Covid-19 is likely to require that 70% of the population develop significant antibodies. The data coming out of Spain and Sweden suggest that a population exposed to Covid-19 through a reopening strategy or natural outbreak is unlikely to achieve even close to the level of infection required for herd immunity to develop. Thus, relying on natural herd immunity alone is unlikely to be an effective strategy for eradicating Covid-19.
 Todd Ellerin has appeared as an ABC medical contributor, is an instructor in medicine at Harvard Medical School, a Medical Technical specialist for Ebola and Sars as well as coronavirus.
Samuel Marks is a full-time student at Swarthmore College, where he is a double major in Economics and History, with a focus on American popular culture. He is co-president of Swarthmore’s Conservative Society and actively volunteers for political campaigns. He has an article pending publication in Fretboard Journal Magazine and plays jazz professionally in the Boston area, where he currently resides.