Thanks to a longitudinal study (Covidiagnostix), funded by the Italian Ministry of Health, we investigated the antibody response, over a 6-month period, in 1110 health-care professionals (HCPs) injected with both doses of the BNT162b2 vaccine (January-February 2021) at the San Raffaele Hospital in Milan, Italy.
Eight HCPs infected after vaccination were female… and four were male…
One individual was infected between the first and second vaccine doses, nine were infected between 7 and 99 days after the second dose and two were oblivious to having being infected.
All the individuals were asymptomatic, except for four who reported partial anosmia and ageusia [loss of smell and taste] accompanied, in three cases, by a mild cold and, in one of those three cases, by a generalized pain.
Seven of the individuals had anti-RBD titres at T2 above 2000 binding antibody units (BAU)/mL, three were between 1000 and 2000 BAU/mL and only two had titres below 400 BAU/mL.
An anti-RBD titre threshold of approximately 1300 BAU/mL was associated with neutralizing activity.
Although the latter is not the only correlate for vaccine efficacy, with memory B and T cells possibly playing a key role in protection, we would have expected a better consistency between high anti-RBD antibody serum levels and protection from infection.
These data further highlight the difficulty of finding a reliable and unique correlate of protection by assessing only the serum neutralizing antibody titres.
It must be noted that two HCPs (individuals 3 and 5) did not respond to the first vaccine dose and showed T1 anti-RBD titres <0.4 U/mL.
[B]ecause some of the HCPs did not undergo anti-N serological testing, the number of infections might be under-estimated.
Our study showed that, in the observed cohort of HCPs, no severe clinical manifestations of coronavirus disease 2019 occurred. We might speculate that the latter is the consequence of the efficacy of the BNT162b2 vaccine, but infection and symptomatology were not related to a low anti-RBD antibody response.