Although both previous infection and vaccination were found to significantly protect against COVID-19 infection, vaccination in previously infected individuals provided additional protection against symptomatic COVID-19 only among those who received the vaccine several months after the initial onset of the disease. These findings were published in Clinical Infectious Diseases.

To evaluate the necessity of vaccination among individuals previously infected with COVID-19, researchers conducted a retrospective cohort study among individuals employed at the Cleveland Clinic in Ohio. Participants with at least 1 positive test result for COVID-19 infection were considered previously infected. The primary outcome was time to COVID-19 infection. Researchers analyzed the cumulative incidence of COVID-19, symptomatic COVID-19, and hospitalizations for COVID-19 for 1 year after COVID-19 vaccines became available in the US.

By the conclusion of the study, 4718 (9%) and 36,922 (71%) of a total of 52,238 participants enrolled were either previously infected with or vaccinated against COVID-19 infection, respectively. Of all participants included in the study, 7851 (15%) were infected with COVID-19 during the study period, of whom 4675 (60%) had symptomatic infections and 133 (1.7%) required hospitalization. Of participants infected with COVID-19 during the study period, 4936 (63%) and 2915 (37%) were infected within either the 11 months prior to or within 1 month after the Omicron variant was first detected in the US, respectively. Of note, the cumulative incidence of COVID-19 infection was significantly increased among unvaccinated participants without prior COVID-19 infection.


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The researchers analyzed the results in a Cox proportional hazards regression model and found that both previous infection and vaccination were independently associated with a significantly decreased risk for incident COVID-19 infection. Among participants without prior COVID-19 infection, vaccination was associated with a significantly decreased risk for incident COVID-19 in both the period prior to the emergence of Omicron (hazard ratio [HR], 0.26; 95% CI, 0.24-0.28) and the period after the emergence of Omicron (HR, 0.48; 95% CI, 0.44-0.53). In patients with prior COVID-19 infection, the researchers noted that vaccination was not associated with a significantly decreased risk for incident COVID-19 in either the period prior to (HR, 0.78; 95% CI, 0.31-1.96) or after the emergence of the Omicron variant (HR, 0.77; 95% CI, 0.53-1.12). Among participants with prior COVID-19 infection, in both the periods prior to and after the emergence of the Omicron variant, vaccination was associated with a significantly decreased risk for symptomatic COVID-19 (HR, 0.60; 95% CI, 0.40-0.90 vs HR, 0.36; 95% CI, 0.23-0.57; respectively).

Based on additional analysis of the results, the researchers estimated the duration of protection against COVID-19 infection conferred by natural immunity among unvaccinated participants to be 1 year or more for the period prior to the emergence of Omicron.  

Study limitations included a lack of routine testing among asymptomatic participants as those previously infected who remained asymptomatic were potentially misclassified as previously infected. In addition, the study included no children, few older adults, and only a small number of participants who were immunocompromised, thus results may not be generalizable to other patient populations.

According to the researchers, “both previously infected individuals and those who [have] been vaccinated are substantially protected against COVID-19 infection, but protection from both natural and vaccine-induced immunity wanes with time and is inherently less potent against the Omicron variant.” They concluded that “previous infection should be factored into COVID-19 vaccination recommendations.”

Reference

Shrestha NK, Burke PC, Nowacki AS, Terpeluk P, Gordon SM. Necessity of COVID-19 vaccination in persons who have already had COVID-19. Clin Infect Dis. Published online January 13, 2022. doi: 10.1093/cid/ciac022

This article originally appeared on Infectious Disease Advisor