Harvard-Affiliated Study of Healthcare Workers Evidences Robust Natural Immunity

Harvard-Affiliated Study of Healthcare Workers Evidences Robust Natural Immunity

A group of Harvard-affiliated researchers monitored both vaccinated and unvaccinated Massachusetts healthcare workers revealing zero infections in 74,557 person-days for those who were previously infected with SARS-CoV-2 as compared to 49 infections out of 830,084 person-days for fully vaccinated patients. This study provides strong evidence as to the robustness of natural immunity. The healthcare workers that were infected previously appear to have greater immunity—the data suggests they aren’t a threat to transmit the pathogen. This runs contrary to the dominant narrative today that only universal vaccination leads to lower transmission rates. The study hasn’t been peer-reviewed as of yet thus shouldn’t be used for evidence until the findings can be substantiated.

The study period ran from December 16, 2020, to September 30, 2021, when 4615 healthcare workers contributed a total of 1,152,486 person-days at risk (excluding 309 health care workers with prior infection) and had a COVID-19 incidence rate of 5.2/10,000 (114 infections out of 219,842 person-days) for unvaccinated person-days and 0.6/10,000 (49 infections out of 830,084 person-days) for fully vaccinated person-days leading adjusted vaccine effectiveness of 82.3% (95% CI: 75.1-87.4%).

The team conducted a secondary analysis during the period that the Dela variant was most common from July 1 to September 30, 2021. The study team observed that the adjusted vaccine effectiveness was 76.5% (95% CI: 40.9-90.6%). 

Importantly the study authors report that independently they found absolute no reinfection among those healthcare workers who had prior COVID-19 infections, contributing to 74,557 re-infection-free person days, “adding to the evidence base for the robustness of naturally acquired immunity.”

Study Population

As reported in the preprint server, medRxiv a total of 4,615 Healthcare workers participated in this study. The average age was 45.0±13.3 years and female predominance (76.0%)) contributed to 1,152,486 person-days at risk during the study period. Forty-five percent of the study population was non-White (including 20% African American, 13.5% Hispanic, and 9.0% Asian).

Of all healthcare workers participating, 4,418 (95.7%) had received at least one dose by the end of the study. Among them, 58.3% got Moderna, 39.4% Pfizer, 2.3% J&J/Janssen, and one (0.02%) got mixed doses of J&J/Janssen and Moderna. The results showed that throughout the study period, for fully vaccinated healthcare workers the vaccine effectiveness is 82.3% (95% CI: 75.1–87.4%) after multivariable adjustment.


The study authors write that to their knowledge this is “one of the fist in healthcare setings regarding continued vaccine effectiveness during delta variant predominance.”   In addition to exposing robust natural immunity the study found vaccine effectiveness of 76% against the delta variant.

Lead Research/Investigator

·       Fan-Yun Lan, Harvard T.H. Chan School of Public Health

·       Amalia Sidossis, post doctoral research fellow, Harvard T.H. Chan School of Public Health

·       Eirini Llaki, Occupational Medicine, Cambridge Health Alliance, Harvard Medical School

·       Jane Buley, Director of Occupational Health, Cambridge Health Alliance

·       Neetha Nathan, Cambridge Health Alliance

·       Lou Ann Bruno-Murtha, DO, Assistant Professor of Medicine, Cambridge Health Alliance

·       Stefanos N. Kales, MD, MPH Professor Department of Environmental Health, Harvard T.H. Chan School of Public Health

Call to Action:  Recovered healthcare workers appear to have robust natural immunity based on these study results. This would imply that those patients that recover from COVID-19 don’t pass it on—even if not vaccinated. It is not clear what happens to this cohort if subsequently immunized. TrialSite community—what are your thoughts?