When vaccines to prevent COVID-19 first became available in the United States in December 2020, the Centers for Disease Control and Prevention (CDC) didn’t distinguish between those who had previously been infected and those who hadn’t — both groups were recommended to receive two doses of mRNA-containing vaccines. Now, however, researchers are beginning to examine the influence of previous infection on vaccine responses.

Researchers from the University of Maryland stratified healthcare workers into three groups: 1) SARS-CoV-2-antibody negative; 2) SARS-CoV-2-antibody positive with previous asymptomatic infection; and 3) SARS-CoV-2-antibody positive with previous symptomatic infection (Saadat S, Tehrani ZR, Logue J, et al. Binding and neutralizing antibody titers after a single vaccine dose in health care workers previously infected with SARS-CoV-2. JAMA. 2021 Apr 13;325(14):1467-1469). After receipt of one dose of mRNA-containing vaccines, they found that the levels of virus-binding and virus-neutralizing titers were significantly greater in those who had been previously infected with the virus than those who hadn’t. Of interest, the levels of antibodies were indistinguishable in those who had been asymptomatically or symptomatically infected. The authors concluded, “Given the ongoing worldwide vaccine shortages, the results inform suggestions for a single-dose vaccination strategy for those with prior COVID-19 …” (p. 1468).

Similarly, researchers in Europe examined about 100 nursing home residents who either did or did not have a previous COVID-19 infection. They found that those with previous infection who received one dose of mRNA vaccine had levels of virus-specific antibodies similar to what would be found after two doses in people not previously infected (Blain H, Tuaillon E, & Gamon L. Spike antibody levels of nursing home residents with or without prior COVID-19 3 weeks after a single BNT162b2 vaccine dose. JAMA. 2021 Apr 15;325(18):1898-1899). These researchers also concluded that “a single dose of BNT162b2 (mRNA) vaccine may be sufficient to obtain a high level of S-protein IgG antibody in nursing home residents previously diagnosed with COVID-19” (p. 1899).

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