Immunization and SARS-CoV-2 Antibody Seroprevalence in a Country with High Vaccination Coverage: Lessons from Chile

Abstract

Chile is among the most successful nations worldwide in terms of its COVID-19 vaccine rollout. By 31 December 2021, 84.1% of the population was fully vaccinated, and 56.1% received booster doses using different COVID-19 vaccines. In this context, we aimed to estimate the prevalence of anti-SARS-CoV-2 antibodies following the infection and vaccination campaign. Using a three-stage stratified sampling, we performed a population-based cross-sectional serosurvey based on a representative sample of three Chilean cities. Selected participants were blood-sampled on-site and answered a short COVID-19 and vaccination history questionnaire using Wantai SARS-CoV-2 Ab ELISA to determine seroprevalence. We recruited 2198 individuals aged 7–93 between 5 October and 25 November 2021; 2132 individuals received COVID-19 vaccinations (97%), 67 (3.1%) received one dose, 2065 (93.9%) received two doses, and 936 received the booster jab (42.6%). Antibody seroprevalence reached 97.3%, ranging from 40.9% among those not vaccinated to 99.8% in those with booster doses (OR = 674.6, 154.8–2938.5). SARS-CoV-2 antibodies were associated with vaccination, previous COVID-19 diagnosis, age group, and city of residence. In contrast, we found no significant differences in the type of vaccine used, education, nationality, or type of health insurance. We found a seroprevalence close to 100%, primarily due to the successful vaccination program, which strongly emphasizes universal access.

SARS-CoV-2 Neutralizing Antibodies in Chile after a Vaccination Campaign with Five Different Schemes

“…In fact, in two of the three subjects without nAbs, the sample collection was before 14 days, and this occurred in just one of the seventeen nAbs responders. The neutralization analysis detected significant differences according to vaccine technologies not seen in measuring total SARS-CoV-2 antibodies [ 11 ]. This added value, provided by neutralization studies, allows a deeper understanding of the antibody response to vaccines and natural infection to guide the public health response to the pandemic.…”

“…Serum neutralization capacity was measured using a pseudotyped vesicular stomatitis virus with a sequence encoding the enhanced green fluorescent protein as a reporter gene (VSV-GFP-Spike SARS-CoV-2 original Wuhan strain), kindly donated by Dr. Kartik Chandran [ 9 ]. Samples tested came from individuals enrolled in a population-based SARS-CoV-2 seroprevalence study performed by the same research team [ 10 , 11 , 12 ]. In November 2021, we collected 2198 serum samples from seven-year-old and older people, finding 97.3% of seropositivity.…”

Seroprevalence of Natural and Acquired Immunity against the SARS-CoV-2 Virus in a Population Cohort from Two Chilean Cities, 2020–2022

“…All population samples were randomly selected and were representative of both cities. The details of the sampling procedures for the first and second phases are described elsewhere [ 5 , 11 , 12 ]. Briefly, two-stage random sampling was carried out, stratified by census districts based on (1) block (random selection) and (2) home (systematic jump); all members 7 years of age or older within the sampled homes were invited to participate.…”

“…Pfizer-BioNTech’s BNT162b2, Oxford-AstraZeneca’s AZD1222 and CanSino Biologics’ Ad5-nCoV vaccines were added during the first year, and, finally, Moderna’s COVID-19 (mRNA-1273) was included by February 2022. A detailed description of the vaccines used in Chile in October 2021 (the time of the second evaluation in this study) is described elsewhere [ 11 , 12 ]. The general Chilean population was offered a second booster with a mRNA vaccine at the time of the third evaluation (April–May 2022).…”

“…These seroprevalence studies observed a decrease in natural immunity over time [ 16 , 17 ]. Moreover, the duration of immunity acquired by vaccines remains uncertain and depends on the type of vaccine and inoculation scheme [ 11 , 12 , 18 ]. It is not always possible to measure both types of immunity (natural and acquired) separately via simple determination of antibodies, as IgG antibodies to the nucleocapsid (IgG anti-N) and the spike proteins (IgG anti-S) must be measured separately [ 19 ].…”