As of October 28, 2021, more than 45 million COVID-19 cases and over 740,000 deaths have been reported in the United States (US). Substantial immunologic evidence and a growing body of epidemiologic evidence indicate that vaccination after infection significantly enhances protection and further reduces risk of reinfection, which lays the foundation for CDC recommendations.ĬDC recommends COVID-19 vaccination for all eligible persons, including those who have been previously infected with SARS-CoV-2.There are insufficient data to extend the findings related to infection-induced immunity at this time to persons with very mild or asymptomatic infection or children. The body of evidence for infection-induced immunity is more limited than that for vaccine-induced immunity in terms of the quality of evidence (e.g., probable bias towards symptomatic or medically-attended infections) and types of studies (e.g., observational cohort studies, mostly retrospective versus a mix of randomized controlled trials, case-control studies, and cohort studies for vaccine-induced immunity).Current evidence indicates that the level of protection may not be the same for all viral variants.For certain populations, such as the elderly and immunocompromised, the levels of protection may be decreased following both vaccination and infection.Whereas there is a wide range in antibody titers in response to infection with SARS-CoV-2, completion of a primary vaccine series, especially with mRNA vaccines, typically leads to a more consistent and higher-titer initial antibody response.Multiple studies have shown that antibody titers correlate with protection at a population level, but protective titers at the individual level remain unknown.The immunity provided by vaccine and prior infection are both high but not complete (i.e., not 100%).At this time, there is no FDA-authorized or approved test that providers or the public can use to reliably determine whether a person is protected from infection. Data are presently insufficient to determine an antibody titer threshold that indicates when an individual is protected from infection. Available evidence shows that fully vaccinated individuals and those previously infected with SARS-CoV-2 each have a low risk of subsequent infection for at least 6 months.Key findings and considerations for this brief are as follows: Vaccine-induced Immune Responses after Previous Infection.
Comparison of Infection- and Vaccine-induced Immune Responses.Impact of Variants on Infection- and Vaccine-induced Immunity.Immune Response Kinetics and Duration of Protection.Correlation of Immune Response Metrics to Protection.Immune Response to Infection and Vaccination.
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