Influence of Pre-Season Antibodies against Influenza Virus on Risk of Influenza Infection among Health Care Personnel.

BACKGROUND The association of hemagglutination inhibition (HAI) antibodies with protection from influenza among healthcare personnel (HCP) with occupational exposure to influenza viruses has not been well-described. METHODS The Respiratory Protection Effectiveness Clinical Trial was a cluster-randomized, multi-site study that compared medical masks to N95 respirators in preventing viral respiratory infections among HCP in outpatient healthcare settings for 5,180 participant-seasons. Serum HAI antibody titers before each influenza season and influenza virus infection confirmed by polymerase chain reaction were studied over four study years. RESULTS In univariate models, the risk of influenza A(H3N2) and B virus infections was associated with HAI titers to each virus, study year, and site. HAI titers were strongly associated with vaccination. Within multivariate models, each log base 2 increase in titer was associated with 15%, 26% and 33-35% reductions in the hazard of influenza A(H3N2), A(H1N1) and B infections, respectively. Best models included pre-season antibody titers and study year, but not other variables. CONCLUSIONS HAI titers were associated with protection from influenza among HCP with routine exposure to patients with respiratory illness and influenza season contributed to risk. HCP can be reassured about receiving influenza vaccination to stimulate immunity.