The results show a high seroprevalence of antibodies in the subjects studied, strongly associated with vaccination, consistent with the immunization coverage registered by the Chilean Ministry of Health for the three cities. At the end of October, during the fieldwork, reported vaccine coverage was 83.8% for the first dose, 76.9% for the complete scheme and 29.9% for the booster jab .
Our results show a higher seroprevalence than that reported in other population-based seroprevalence studies. In India between June and July 2021, seropositivity in adults was 66.7% ; in Switzerland, in the same period, it reached 64.4%, with 30% due to natural infection , while in Greece in June 2021, it was 55.7% . This difference may be due to the high vaccination coverage achieved in Chile. On the other hand, the study carried out in India, similar to our results, shows an antibody gradient according to the number of vaccine doses (62% among those not vaccinated; 81% with one vaccine dose, and 89.8% with a full scheme) .
Coquimbo-La Serena showed a significantly lower seroprevalence than the other two cities, which could be explained by the higher proportion of unvaccinated individuals in that city obtained in our study (5%, versus 2.5% in Santiago and 2% in Talca). Another factor could be that the accumulated incidence in Coquimbo was lower than that in the other cities at the time of the study (7,059 per 100.000 inhabitants; versus 11,724 in Santiago and 10,076 in Talca).
The lower seroprevalence observed in children under 10 years of age is probably due to their late incorporation into the vaccination campaign, which as of September 27th, 2021 covered children between 6 and 11 years of age. We also found an association between seroprevalence and history of COVID-19, but with lower significance than vaccination.
The lack of association of seroprevalence with education, health insurance (public or private) and nationality is consistent with the strategy of the health authority. The early purchase of jabs and the implementation of a universal vaccination campaign allowed access to all people, taking care of vulnerable and hard-to-reach populations, including migrants. In contrast, the Geneva study showed significant differences in vaccination coverage related to education, leading to higher seroprevalence among more educated people .
The seroprevalence among vaccinated (any dose) reached 99.1%, while that among unvaccinated was 40.9%. We observed a gradient effect between the prevalence of antibodies and the number of vaccine doses received. Those with the booster doses reach almost 100% seroprevalence, independent of the type of vaccine used.
Regarding the type of vaccines applied, the most significant antibody variation occurred among those who received only one dose. In contrast, those who completed the two-dose schedules had a similar seroprevalence, regardless of the type of vaccine administered, and was even more homogeneously high in those with booster doses.
The prevalence of antibodies in unvaccinated people demonstrates the burden of infection achieved with successive pandemic waves. In our previous survey, at the end of 2020, the seroprevalence was 10.4%, with differences between the three cities. There are still differences among the cities, although all of them have very high seroprevalence.
Our study had some limitations. The rate of rejection of former participants could be associated with the vaccination status; therefore, our results might have shown higher vaccine uptake than officially reported. However, this does not preclude comparative analysis between vaccinated, unvaccinated and the different vaccine schemes. Further analysis will include weighted seroprevalence and neutralizing antibody responses.
In sum, our results show a prevalence of antibodies close to 100% in the population aged seven years and older, primarily due to the successful vaccination program, which has a strong emphasis on universal access. We also observed that seroprevalence remained very high 180 days after the last vaccine dose. However, it might not be enough to prevent the recent summer resurgence of cases resulting from the circulation of the Omicron variant, even though it is expected to have less impact on hospitalizations and deaths. We still need to learn about SARS-CoV-2 transmission, the protection conferred by vaccines, and their relationship with the evolution of the virus.