Published On: March 25, 2024

New results from a cohort of Catalan healthcare workers support prioritising boosters for people who have not been recently infected  

Photo: @franciscoavia Hospital Clínic Barcelona

Antibodies produced in response to COVID-19 vaccines targeting the original variant, as well as those generated through a combination of vaccination and infection, are valid correlates of protection against Omicron, despite its ability to better evade immunity. This is the main conclusion of a cohort study led by END-VOC researchers at the Barcelona Institute for Global Health (ISGlobal). The findings, published in BMC Medicine, also support the need to prioritise booster vaccinations in people without recent infections.

Protection associated with higher antibody levels

The protection provided by COVID-19 vaccines depends not only on the duration of vaccine-induced immunity, but also on the emergence of new viral variants. Before Omicron, it became clear that the level of vaccine-induced antibodies correlated well with protection against infection. “However, it was not clear whether this would hold true with Omicron, which is very different from the earlier variants,” explains Carlota Dobaño, Head of the Immunology Group at ISGlobal and co-senior author of the study together with colleague Gemma Moncunill. It also wasn’t clear whether hybrid immunity, resulting from previous infection plus vaccination, would provide additional protection against the new variant – an important element for informing public health strategies and for optimising individual protection.

In this study, Dobaño and her team assessed the antibody response to the Spike (S) and Nucleocapsid (N) proteins of Omicron BA.1 in a cohort of healthcare workers from the Hospital Clinic (SEROCOV) that they have been following since the early days of the pandemic. The study involved 393 participants, of whom 287 had received a booster (or third dose) one year before, and 243 had been previously infected with SARS-CoV-2.

The results show that higher levels of anti-Spike IgG and IgA to the original (Wuhan), Delta and Omicron BA.1 variants were associated with protection against infection by Omicron subvariants.

And recent infections

But recent infections provided the strongest protection, regardless of antibody levels. The authors point out that this may be due not only be a matter of time, but also of the infecting variant, as the most recent infections were with Omicron, while older infections were with pre-Omicron variants.

Overall, this information may help to optimise vaccination strategies and to prioritise booster doses for those who have not been recently infected.



Martin Perez C, Aguilar R, Jiménez A et al. Correlates of protection and determinants of SARS-CoV-2 breakthrough infections one year after third dose vaccination. BMC Med. 2024. doi:10.1186/s12916-024-03304-3.