Hospitalisation among vaccine breakthrough COVID-19 infections

172 (18%) of 969 patients had received at least one dose of a COVID-19 vaccine at the time of admission to hospital. Among these patients, 103 had received a partial vaccine course (one dose of BNT162b2 or mRNA-1273), 15 had received a complete course (two doses of BNT162b2 or mRNA-1273 or one dose of Ad.26.COV2.S within 14 days before symptom onset or a positive PCR test), and 54 were fully vaccinated (appendix pp 1–2). Patients deemed to have a breakthrough SARS-CoV-2 infection—ie, the 54 patients who were fully vaccinated—were evaluated for illness severity. Among this cohort, we found that 25 (46%) patients were asymptomatic (admitted to hospital for a non-COVID-19-related diagnosis but with an incidental positive PCR test for SARS-CoV-2), four (7%) had mild disease, 11 (20%) had moderate disease, and 14 (26%) had severe or critical illness. Among those with severe or critical illness, the median age was 80·5 years (IQR 76·5–85·0); four of 14 patients required intensive care, one required mechanical ventilation, and three died. Pre-existing comorbidities in the 14 patients with severe or critical illness included overweight (body–mass index >25 kg/m²; n=9), cardiovascular disease (n=12), lung disease (n=7), malignancy (n=4), type 2 diabetes (n=7), and use of an immunosuppressive agent (n=4; appendix pp 3). 13 of 14 patients had received BNT162b2 (appendix p 1–2).

Vaccination for SARS-CoV-2 is highly effective against infection with SARS-CoV-2 or hospitalisation with COVID-19. In our real-world assessment of patients admitted to hospital with a positive SARS-CoV-2 PCR test, we found that nearly a fifth of patients had received at least one dose of the vaccine, and we observed that many patients had not completed the full vaccine course. The finding that more than a quarter of fully vaccinated patients admitted to hospital with SARS-CoV-2 were severely or critically ill with COVID-19 could be reflective of numerous factors, including the emergence of SARS-CoV-2 variants that might confer decreased vaccine effectiveness and an ineffective immune response mounted against vaccines among those with comorbidities—eg, older age, overweight, and use of immunosuppressive agents. Although the incidence of severe or critical COVID-19 illness remains low in those who are fully vaccinated, we observed a higher number of patients with severe or critical illness in those who received the BNT162b2 vaccine than in those who received mRNA-1273 or Ad.26.COV2.S (total number of vaccine doses distributed in Connecticut [USA] until May 17, 2021, was 1 358 175 for BNT162b2, 1 044 420 for mRNA-1273, and 267 000 for Ad.26.COV2.S).

This finding would benefit from further investigation. Overall, although vaccines have undoubtedly conferred widespread protection against SARS-CoV-2 infection worldwide, future studies are needed to identify and mitigate factors that are associated with inadequate vaccine response in those with breakthrough infections.