Neutralization and Anti-S Antibody Titers in SARS-CoV-2 Boosted Adults in Mexico: A Comparison Across Five Vaccine Types.

The World Health Organization (WHO) recommends the administration of booster doses after completing primary vaccination. Yet, recommendations are largely based on a small set of vaccines, with little information about vaccine mixes used in low and middle-income countries. We aimed to estimate the titers of anti-protein S (RBD) IgG antibodies and neutralization in Mexican adults with a complete vaccination scheme without a booster or with one of five different booster types (heterologous or homologous) to assess immunogenic response. We included data from 2953 adults aged 18 years and older, representing 49.1 million Mexicans, from the National Health and Nutrition Survey (ENSANUT) 2022. We included five primary schemes: BNT162b2, AZD1222, Ad5-nCoV, Gam-COVID-Vac, and CoronaVac. A booster dose was defined as an additional dose beyond the primary scheme and classified as homologous or heterologous based on three vaccine platforms: mRNA, viral vector, and inactivated virus. For each primary scheme, we estimated marginal means of anti-protein S geometric mean titers (GMTs) with 95% CI in each group (no booster, heterologous booster, or homologous booster) adjusted for age, time since the last dose, and prior natural infection using anti-protein N antibodies seropositivity as a proxy using a multivariable linear regression. Then, we estimated the percentage of neutralizing antibodies for the original SARS-CoV-2 strain. We observed that the highest anti-protein S GMTs were in the group that received a heterologous booster with BNT162b2, AZD1222, Ad5-nCoV, or CoronaVac; the group without a booster showed the lowest GMTs. All groups in the primary schemes had 90% or more inhibition for the original SARS-CoV-2 strain, except in the AZD1222 and Ad5-nCoV groups without a booster. Boosters increased GMTs for all people, independently of their primary vaccine scheme. Yet, our findings suggest that a heterologous booster produced higher titers against SARS-CoV-2 protein S. Efforts should be made to reach people who received AZD1222 and Ad5-nCoV as their primary scheme and did not get boosted, as neutralization in those groups was particularly low.