To determine the percentage of the population that has been either naturally infected with SARS-CoV-2 virus or immunized with SARS-CoV-2 vaccines, researchers from the Centers for Disease Control and Prevention (CDC) evaluated blood donations from all 50 states (Jones JM, Stone M, Sulaeman H, et al. Estimated US infection- and vaccine-induced SARS-CoV-2 seroprevalence based on blood donations, July 2020-May 2021. JAMA. 2021 Oct 12;326(14):1400-1409). About 1.5 million blood specimens were examined. Sera containing antibodies against the SARS-CoV-2 nucleoprotein were considered to have come from people who had been naturally infected, whereas sera containing antibodies to the SARS-CoV-2 spike protein but not the nucleoprotein were considered to have come from people who had been vaccinated but not naturally infected.

The authors found that the overall infection-induced SARS-CoV-2 seroprevalence rose from 3.5% in July 2020 to 20.1% in May 2021. The combined infection- and vaccination-induced seroprevalence estimate in May 2021 was 83.3%. Both infection and immunization likely induce protective immunity. These findings show that 83% population immunity is not enough to significantly slow this pandemic.  

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