Key Points

After more than two years of low incidence, linked to measures implemented during the COVID-19 pandemic, the number of IIM cases has been rising again since October 2022.

• With 84 cases of IIM reported in December 2022, the incidence is at a high level and has exceeded the monthly peak typically observed later in the winter seasons prior to the COVID-19 pandemic (peak between January and March, depending on the season).

• In 2022, the vast majority of IIM cases were linked to serogroups B (53% of cases), Y (23% of cases), and W (19% of cases), while serogroup C accounted for a very small proportion (3% of cases).

• The distribution of cases by age and serogroup shows that:

  • IIM B cases primarily affected young adults aged 15–24 and infants, as observed before the COVID-19 pandemic. Compared to the pre-pandemic period, there was a more pronounced increase in IIM B cases in 2022 among those aged 15–24;

  • Y-serogroup IIMs remain more common among adults than among children, particularly among those aged 60 and older. Compared to the pre-pandemic period, the Y serogroup caused a higher number of invasive infections among young adults aged 15–24, while in other age groups, the number of cases was comparable to pre-pandemic levels;

  • W-serogroup IIMs have also been on the rise since November across several age groups (infants, young adults aged 15–24, and older adults). According to strain characterization data from the National Meningococcal Reference Center, serogroup W strains are predominantly linked to ST (sequence type) 9316 and clonal complex 11. ST-9316 strains, which were in the minority before the pandemic, appear to have spread throughout the population.

• In 2022, 10% of deaths were reported among IIM cases, representing a case fatality rate equivalent to that observed before the pandemic, with variations by serogroup and age. The case fatality rate for W IIM was high in 2022 but was equivalent to the rate observed before the pandemic (21% in 2022 vs. 21.8% between 2016 and 2019).