An epidemiological survey of COVID-19 serology and its association with clinical infection among older adults- does antibody titer matter?

ABSTRACT

BACKGROUND: Older adults are at increased risk of severe SARS-CoV-2 infection. In this study we assessed the response to COVID-19 vaccination and infection rates among nursing homes (NH) and assisted-living care home (ALCH) residents. METHODS: The study was conducted between August 2021 and January 2022, after widespread population vaccination with the third dose of Pfizer-BioNtech mRNA COVID-19 vaccine in Israel. Three groups were addressed hospitalized older patients; NH and ALCH residents. Demographic data, COVID-19 serology (anti-spike IgG antibodies) and PCR test results were obtained to assess the dynamics of antibody titers and its correlation to infection rates. RESULTS: Two-hundred eighty-five individuals were evaluated; 92 hospitalized patients; 100 ALCH residents and 93 NH residents. In the latter two groups two serology surveys were conducted three months apart. Hospitalized patients were younger than ALCH and NH residents (mean age 80.4 ± 8 versus 82.6 ± 8 and 83.6 ± 5, respectively, p = 0.01), and had more comorbidities (p = 0.003). The degree of decline in the antibody level overtime was similar in ALCH and NH residents. Infection rates were higher among NH residents than ALCH residents [35/91 (38.4%) versus 11/100 (11%), p < 0.001]. Antibody level was lower among those infected [2113 (1271-3512) Au/ml versus 4113 (3364-5029) Au/ml, p < 0.001]. Adjusted analysis showed that NH residence, but not antibody levels, were significantly associated with infection. CONCLUSION: Among older adults, infection rates inversely correlated with antibody level. However, only nursing home residence was significantly associated with infection, suggesting that other factors such as crowding considerably contribute to the risk of infection.