LAUSR

Importance: Scarce information exists concerning the seroconversion and adverse events after immunization (AEFI) of the fourth dose of a SARS-COV-2 vaccine. Objective: Correlate the magnitude of the antibody response to vaccination with previous clinical conditions and AEFI of the fourth dose of BNT162b2 mRNA. Design: Observational study where SARS-CoV-2 spike 1-2 IgG antibodies IgG titers were measured 21-28 days after the exposition of the first, and second dose, three months after the second dose, 1-7 days after the third dose, before the fourth dose, and 21-28 days after the fourth dose of BNT162b2 mRNA. Setting: The study was conducted on healthcare workers of a private hospital in Northern Mexico. Participants: Inclusion criteria were healthcare workers of both genders, any age, who planned to conclude the immunization regimen. The exclusion criteria were previously given any SARS-CoV-2 vaccine prior to study entry. Intervention: Subjects were exposed to four doses of the BNT162b2 mRNA vaccine. Main Outcome and Measures: The anti-S1 and anti-S2 IgG antibodies against SARS-CoV-2 in plasma samples were measured with chemiluminescence immunoassay developed by DiaSorin. Results: We recruited 112 subjects [43 (SD 9) years old, 74% women]. After the first dose, subjects had a median (IQR) AU/ml IgG of 122(1904), with an increase to 1875 (2095) after the second dose, 3020 (2330) after the third dose, and 4230 (3393) after 21-28 of a fourth dose (p<0.01). The number (%) of any AEFI between doses was 90 (80.4), 89(79), 65(58), 69 (61.5), after first, second, third, and fourth, respectively, p<0.001. After the fourth dose, the most frequent AEFI was pain at the injection site (87%). Fever was slightly more frequent after the third and fourth doses, 9 (13.8) and 8 (11.4%) cases, respectively, and adenopathy was more frequent after the fourth dose [in11(15.7%) cases]. There was a correlation between AEFI in the fourth dose with gender and antibody levels (p<0.05). The highest proportion of AEFI was considered mild after the fourth dose. During the Omicron outbreak, 6 (5.3%) had mild SARS-CoV-2 during 8-28 days of the fourth dose. Conclusions and Relevance: The fourth dose of BNT162b2mRNA increases S1/S2 IgG 33.6 times with mild adverse events.