Objective: To determine if ABO blood group, age, body mass index (BMI), or symptomatic COVID-19 infection are associated with COVID-19 antibody response in unvaccinated COVID-19 antibody positive pregnant women at time of delivery.

Methods: At the time of delivery, 2,499 consecutive pregnant women were tested for ABO blood group and antibodies to both the spike protein and nucleocapsid protein of the COVID-19 virus. The DiaSorin assay was used for antibody to spike protein analysis and the Roche assay for antibody to nucleocapsid protein. Gamma regression models with a log link were used to compare antibody signals, with blood group, age, and BMI as the predictors.

Results: 260 (10.4%) of 2,499 women who had not been vaccinated for COVID-19, were positive for both spike and nucleocapsid protein antibodies to COVID-19. The mean signal for COVID-19 nucleocapsid antibody was significantly lower for blood group AB (p=0.028) compared with blood group O. A relationship between blood group and presence of symptomatic COVID-19 was detected (p = 0.028), with asymptomatic individuals having blood group B at a higher rate than the symptomatic individuals do.

No other significant pairwise differences between blood groups were detected. There was no significant difference in signal level of antibodies to COVID-19 spike protein between any of the blood groups. Mean signals for antibodies to spike and nucleocapsid proteins were significantly higher in older women (p=0.001 for spike protein antibody and p=0.002 for nucleocapsid antibody). Significantly higher signal levels of antibody to spike and nucleocapsid proteins were found in women with class 2/3 obesity (p=0.022 and p=0.003, respectively).

Conclusions: Pregnant women of AB blood group had lower antibody signal to nucleocapsid protein compared to the O blood group, and women of older age and greater BMI had higher antibody signal to COVID-19 spike and nucleocapsid proteins. There was a significant association between women with blood group B and asymptomatic infections.