Early Omicron infection is associated with increased reinfection risk in older adults in long-term care and retirement facilities.

Background: Older adults are at increased risk of SARS-CoV-2 Omicron infection and severe disease, especially those in congregate living settings, despite high SARS-CoV-2 vaccine coverage. It is unclear whether hybrid immunity (combined vaccination and infection) after one Omicron infection provides increased protection against subsequent Omicron reinfection in older adults.
Methods: Incidence of SARS-CoV-2 Omicron infection was examined in 750 vaccinated residents of long-term care and retirement homes in the observational cohort COVID in Long-Term Care Study in Ontario, Canada, within a 75-day period (July to September 2022). Risk of infection was assessed by Cox proportional hazards regression. Serum anti-spike and anti-RBD SARS-CoV-2 IgG and IgA antibodies, microneutralization titres, and spike-specific T cell memory responses, were examined in a subset of 318 residents within the preceding three months.
Findings: 133 of 750 participants (17.7%) had a PCR-confirmed Omicron infection during the observation period. Increased infection risk was associated with prior Omicron infection (at 9-29 days: 47.67 [23.73-95.76]), and this was not attributed to days since fourth vaccination (1.00 [1.00-1.01]) or residence outbreaks (>6 compared to ≤6: 0.95 [0.37-2.41]). Instead, reinfected participants had lower serum neutralizing antibodies to ancestral and Omicron BA.1 SARS-CoV-2, and lower anti-RBD IgG and IgA antibodies, after their initial Omicron infection.
Interpretation: Counterintuitively, SARS-CoV-2 Omicron infection was associated with increased risk of Omicron reinfection in residents of long-term care and retirement homes. Less robust humoral hybrid immune responses in older adults may contribute to risk of Omicron reinfection.
Funding: COVID-19 Immunity Task Force of the Public Health Agency of Canada.
Competing Interests: TK has received funding from the COVID-19 Immunity Task Force of the Public Health Agency of Canada to attend a conference of the COVID-19 Immunity Task Force. MSM has received research grants from Providence Therapeutics, Medicago, Bay Area Health Trust, and Lactiga, and honoraria for speaking at Boehringer Ingelheim and development of COVOICES Educational Initiatives at Sanofi, as well as consulting fees from Seqirus, Sanofi, and Grifols, has patents in unrelated technology, holds stock with Aeroimmune Biotherapeutics and stock options with Kapoose Creek Wellness, and is a member of the COVID-19 working group of the National Advisory Committee on Immunization (NACI). APC has received other research funding from the COVID-19 Immunity Task Force of the Public Health Agency of Canada. DMEB has received honoraria from AstraZeneca Mexico for a lecture, consulting fees from Pfizer and AstraZeneca, and holds a volunteer position on the Board of Directors of the Ontario Lung Health Foundation. All other authors declare no competing interests.
(© 2023 The Author(s).)