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Background: Immunoassays for determining past SARS-CoV-2 infection have not been systematically evaluated in vaccinated persons in comparison with unvaccinated persons.Objective: To evaluate antinucleocapsid antibody (anti-N Ab) seropositivity in mRNA-1273 (Moderna) vaccinees with breakthrough SARS-CoV-2 infection.Design: Nested substudy of a phase 3 randomized, double-blind, placebo-controlled vaccine efficacy trial. (ClinicalTrials.gov: NCT04470427).Setting: 99 sites in the United States, July 2020 through March 2021.Participants: Participants were aged 18 years or older, had no known history of SARS-CoV-2 infection, and were at risk for SARS-CoV-2 infection or severe COVID-19. Substudy participants were diagnosed with SARS-CoV-2 infection during the trial's blinded phase.Intervention: 2 mRNA-1273 or placebo injections 28 days apart.Measurements: Nasopharyngeal swabs from days 1 and 29 (vaccination days) and from symptom-prompted illness visits were tested for SARS-CoV-2 via polymerase chain reaction (PCR). Serum samples from days 1, 29, and 57 and the participant decision visit (PDV, when participants were informed of treatment assignment; median day 149) were tested for anti-N Abs by the Elecsys immunoassay.Results: Among 812 participants with PCR-confirmed COVID-19 illness during the blinded phase of the trial (through March 2021), seroconversion to anti-N Abs (median of 53 days after diagnosis) occurred in 21 of 52 mRNA-1273 vaccinees (40% [95% CI, 27% to 54%]) versus 605 of 648 placebo recipients (93% [CI, 92% to 95%]). Each 1-log increase in SARS-CoV-2 viral copies at diagnosis was associated with 90% higher odds of anti-N Ab seroconversion (odds ratio, 1.90 [CI, 1.59 to 2.28]).Limitation: The scope was restricted to mRNA-1273 vaccinees and the Elecsys assay, the sample size was small, data on Delta and Omicron infections were lacking, and the analysis did not address a prespecified objective of the trial.Conclusion: Vaccination status should be considered when interpreting seroprevalence and seropositivity data based solely on anti-N Ab testing.Primary Funding Source: National Institute of Allergy and Infectious Diseases of the National Institutes of Health. [ABSTRACT FROM AUTHOR]

Multiple candidate vaccines to prevent COVID-19 have entered large-scale phase 3 placebo-controlled randomized clinical trials, and several have demonstrated substantial short-term efficacy. At some point after demonstration of substantial efficacy, placebo recipients should be offered the efficacious vaccine from their trial, which will occur before longer-term efficacy and safety are known. The absence of a placebo group could compromise assessment of longer-term vaccine effects. However, by continuing follow-up after vaccination of the placebo group, this study shows that placebo-controlled vaccine efficacy can be mathematically derived by assuming that the benefit of vaccination over time has the same profile for the original vaccine recipients and the original placebo recipients after their vaccination. Although this derivation provides less precise estimates than would be obtained by a standard trial where the placebo group remains unvaccinated, this proposed approach allows estimation of longer-term effect, including durability of vaccine efficacy and whether the vaccine eventually becomes harmful for some. Deferred vaccination, if done open-label, may lead to riskier behavior in the unblinded original vaccine group, confounding estimates of long-term vaccine efficacy. Hence, deferred vaccination via blinded crossover, where the vaccine group receives placebo and vice versa, would be the preferred way to assess vaccine durability and potential delayed harm. Deferred vaccination allows placebo recipients timely access to the vaccine when it would no longer be proper to maintain them on placebo, yet still allows important insights about immunologic and clinical effectiveness over time. [ABSTRACT FROM AUTHOR]