• Branko Kolarić, Andreja Ambriović-Ristov, Irena Tabain, and Tatjana Vilibić-Čavlek.
• Branko Kolarić, Andrija Štampar Teaching Institute of Public Health, Mirogojska cesta 16, 10000 Zagreb, Croatia, branko.kolaric@gmail.com.
• Croat. Med. J. 2021 Dec 31; 62 (6): 630633630-633.

AimTo assess the humoral immunity to COVID-19 in nursing home residents six months after vaccination.MethodsThis seroepidemiological research enrolled 118 residents of one nursing home in Zagreb. All participants received two doses of BioNTech/Pfizer COVID-19 and had no previously detected SARS-CoV-2 infection. The samples were tested for the presence of neutralizing antibodies using a virus neutralization test. A SARS-CoV-2 strain isolated in Vero E6 cells from a Croatian COVID-19 patient was used as a stock virus. Neutralizing antibody titer was defined as the reciprocal of the highest serum dilution that showed at least 50% neutralization. Neutralizing antibody titer ≥8 was considered positive.ResultsSixty-four (54%) participants had a positive neutralizing antibody titer, 27 (23%) had a low positive titer (titer 8), and 27 (23%) had a negative titer. Women had a significantly higher median titer than men (16 [interquartile range, IQR 24] vs 8 [IQR 12], Mann-Whitney U=1033, P=0.003). Age was negatively but not significantly correlated with neutralizing antibody titer (Spearman's rho -0.132, P=0.155).ConclusionAlmost half of the participants (46%) had a negative or low positive titer six months after having been fully vaccinated. This study suggests that humoral immunity among nursing home residents considerably wanes six months after BioNTech/Pfizer COVID-19 vaccination. Our results could contribute to the discussion about the need for a booster dose.

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