疫学：オミクロン以前と以後のSARS-CoV-2に対する免疫反応

The protective effects of natural infection against repeated exposure to SARS-CoV-2 reduced substantially after the emergence of the Omicron variant, a study in Nature reports. A decline was observed in both the level of protection and duration of immunity in the post-Omicron era compared with the pre-Omicron era. These findings highlight the importance of updating vaccines to maintain immunity.

SARS-CoV-2 has evolved rapidly, producing variants with increased transmissibility. The emergence of the Omicron variant in late 2021 marked a major shift; this variant had more genetic mutations in the spike protein (the region of the virus that facilitates entry into cells) than previous variants. Such rapid evolutionary changes can affect the strength of the immune response to reinfection and may reduce the effectiveness of existing vaccines and treatments.

To understand how viral evolution affects the level and duration of protection provided by natural immunity, Laith Abu-Raddad and colleagues studied population-wide data from individuals in Qatar. Before the Omicron variant, previous infection with SARS-CoV-2 was associated with prolonged and effective protection against reinfection (81.3% effectiveness for the first year after infection, dropping to 79.5% thereafter). However, following the emergence of Omicron, the effectiveness of natural infection at protecting against reinfection was 59.5% for the first year, waning rapidly to 4.8% after one year. The authors suggest that the observed differences in the protective effects of natural infection before and after the Omicron variant emerged indicate that the evolution of the virus may have been driven by the need for immune escape.

The work demonstrates the value of monitoring viral evolution and studying the effects that changes have on host immunity, which can inform vaccine updates to restore immunity, the authors conclude.