Background: Severe acute respiratory syndrome coronavirus 2 immunity has declined with subsequent waves and accrual of viral mutations. In vitro studies raise concern for immune escape by BA.4/BA.5, and a study in Qatar showed moderate protection, but these findings have yet to be reproduced.
Methods: This retrospective cohort study included individuals tested for coronavirus disease 2019 by polymerase chain reaction during Delta or BA.1/BA.2 and retested during BA.4/BA.5. The preventable fraction (PF) was calculated as ratio of the infection to the hospitalization rate for initially positive patients divided by the ratio for initially negative patients, stratified by age and adjusted for age, sex, comorbid conditions, and vaccination using logistic regression.
Results: A total of 20 987 patients met inclusion criteria. Prior Delta infection provided no protection against BA.4/BA.5 infection (adjusted PF, 11.9% [95% confidence interval, .8%-21.8%]); P = .04) and minimal protection against hospitalization (10.7% [4.9%-21.7%]; P = .003). In adjusted models, prior BA.1/BA.2 infection provided 45.9% (95% confidence interval, 36.2%-54.1%; P < .001) protection against BA.4/BA.5 reinfection and 18.8% (10.3%-28.3%; (P < .001) protection against hospitalization. Up-to-date vaccination provided modest protection against reinfection with BA.4/BA.5 and hospitalization.
Conclusions: Prior infection with BA.1/BA.2 and up-to-date vaccination provided modest protection against infection with BA.4/BA.5 and hospitalization, while prior Delta infection provided minimal protection against hospitalization and none against infection.
Keywords: BA5; COVID-19; SARS-CoV-2; epidemiology; immunity; vaccine effectiveness.
© The Author(s) 2023. Published by Oxford University Press on behalf of Infectious Diseases Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.