Almost two years into the Covid-19 pandemic, the question still remains: When will the pandemic end? Some believe the answer to this question lies in herd immunity. With a highly infectious disease such as Covid-19, many have argued that eradication of the virus will occur naturally once it has spread throughout the entire community, generating antibody-mediated immunity in those that recover from the disease. The reasoning is that Covid-19 will simply fade away when there are no remaining vulnerable hosts to infect. Individuals who have argued from the herd immunity perspective often advocate for the continued opening of businesses as well as loosened restrictions for mask-wearing, social distancing, and other preventative measures. These practices, they argue, serve to simply prolong the amount of time it takes to reach herd immunity. Countries like Sweden, which have remained fairly open throughout the pandemic and now reportedly have the virus “under control,”1 have been used to substantiate this argument. The availability of Covid-19 vaccines also raises the question of whether immunity gained from the shots differs from that gained from prior infection.
Covid-19 vaccines are readily available in many high-income countries, including the United States. Many are asking: Is complete vaccine compliance necessary when, at the end of 2020, approximately 31 percent of the U.S. population had already been infected with Covid-19?2 Ostensibly, this would mean that roughly a third of Americans already had some sort of antibody-mediated immunity to the disease, and certainly more have been infected since the publication of the aforementioned study.
Authors Bozio et al. sought to answer this exact question in a study published in November of 2021.3 They examined a number of hospitalized individuals over the age of 18 who had either previously tested positive for Covid-19 infection (via a rapid assay test or reverse PCR testing) or been fully vaccinated with an mRNA vaccine (two doses, given within the recommended timeframe) in the past 90-179 days. The primary outcome was the result of a Covid-19 test taken at the time of hospitalization. When the results were adjusted for pertinent sociodemographic and health factors, the authors found that individuals who had received the Covid-19 vaccine were less likely to test positive for Covid-19 than those who had been previously infected. Interestingly, further analysis revealed that recipients of the Moderna vaccine appeared to have improved immunity when compared to recipients of the Pfizer-BioNTech vaccine. This was consistent with a previous study published in September of 2021 which reported similar findings.4
These accumulative results would suggest that vaccines confers greater immunity when compared to a prior history of Covid-19 infection. While there are some limits to this study – for example, children were excluded, as the U.S. only recently gave full approval for administration of the Pfizer vaccine to individuals aged five and up5 – the results were fairly conclusive. These findings support the CDC recommendation that all Americans eligible to receive the vaccine should do so, for the benefit of both personal and public health.
 Carlsson, M., & Söderberg-Nauclér, C. (2021). Indications that Stockholm has reached herd immunity, given limited restrictions, against several variants of SARS-COV-2. (preprint). https://doi.org/10.1101/2021.07.07.21260167.
2 One in three Americans Already Had COVID-19 by the End of 2020. Columbia Public Health. (2021, August 26). Retrieved from https://www.publichealth.columbia.edu/public-health-now/news/one-three-americans-already-had-covid-19-end-2020.
3 Centers for Disease Control and Prevention. (2021, November 4). Laboratory-Confirmed COVID-19 Among Adults Hospitalized with COVID-19–Like Illness with Infection-Induced or mRNA Vaccine-Induced SARS-COV-2 Immunity – Nine States, January–September 2021. Centers for Disease Control and Prevention. Retrieved from https://www.cdc.gov/mmwr/volumes/70/wr/mm7044e1.htm?s_cid=mm7044e1_w.
4 Self WH; Tenforde MW; Rhoads JP; Gaglani M; Ginde AA; Douin DJ; Olson SM; Talbot HK; Casey JD; Mohr NM; Zepeski A; McNeal T; Ghamande S; Gibbs KW; Files DC; Hager DN; Shehu A; Prekker ME; Erickson HL; Gong MN; Mohamed A; Henning DJ; Steingrub JS; Peltan ID; Brown SM; Martin ET; Mo. (2021). Comparative effectiveness of Moderna, Pfizer-BioNTech, and Janssen (Johnson & Johnson) Vaccines in Preventing COVID-19 Hospitalizations Among Adults Without Immunocompromising Conditions – United States, March-August 2021. MMWR. Morbidity and mortality weekly report. Retrieved from https://pubmed.ncbi.nlm.nih.gov/34555004/.
5 Centers for Disease Control and Prevention. (n.d.). Covid-19 Vaccines for Children and Teens. Centers for Disease Control and Prevention. Retrieved from https://www.cdc.gov/coronavirus/2019-ncov/vaccines/recommendations/children-teens.html