Incidence of MIS-C during the Omicron wave: Authors' replay.

Dear Editor,

We warmly thank Bellini et al. for their comment on our recent paper about Multisystem Inflammatory Syndrome (MIS‐C) and Omicron wave. The authors reported data collected in Paediatric Emergency Department (PED) of Gaslini Children Hospital in Genoa (Italy), showing a statistically significant increase of MIS‐C incidence during the Omicron wave (December 2021–March 2022) to 106.2 cases per 100,000 visits (p = 0.0001), doubled from the previous COVID‐19 peak (54.9 cases per 100,000). On the other hand, they highlight a decrease in incidence of MIS‐C in relation to admissions in their PED with COVID (5080 MIS‐C every 100,000 in April 2020–November 2021 vs. 1439 MIS‐C every 100,000 in December 2021–March 2022, p < 0.0001), maybe because of major COVID‐19 diagnoses in asymptomatic children screened with antigen swab in PED.

Since December 2021, the literature on this topic has not been greatly enriched. Currently, other authors have questioned the possible increase of MIS‐C with the spread of the Omicron variant, partly supporting the data emerging from the Gaslini Hospital PED and our previous hypotheses or refuting them.

In fact, Miller et al. analysed 5670 cases of MIS‐C observed between July 2021 and January 2022 in the United States, dividing them between the third and fourth waves of COVID‐19 and stating that most of the cases (95%) recorded during the fourth wave were in any case due to the Delta variant. In addition, the severity of MIS‐C cases reported during fourth wave and the hospitalisation rate were significantly lower than during third wave. Other interesting data are those published by Levy et al. in May 2022. In this prospective study, the authors compare MIS‐C cases collected in 12 hospitals in Israel during the three waves of COVID‐19, Alpha (December 2020 to April 2021), Delta (July 2021 to November 2021) and Omicron (November 2021 to March 2022). They demonstrate a reduction in the incidence of MIS‐C during the Omicron wave (incidences per 100,000 children were 54.5 during Alpha, 49.2 during Delta and 3.8 during Omicron) and a lower severity of MIS‐C cases. These findings would be in total disagreement with what was found in Italy by Bellini et al., but a different territorial spread of the variants should be considered. Anyway, Bellini et al. described only the incidence of MIS‐C in their hospital during the Omicron wave, without mentioning the disease severity or the rate of hospitalisation.

The high prevalence of SARS‐CoV‐2 infection in children in Italy during the Omicron wave would justify the increase in MIS‐C cases compared to the previous waves. However, the low virulence of the Omicron variant itself, a possible previous SARS‐COV‐2 infection but above all the spread of COVID‐19 vaccination in children aged between 5 and 18 years could justify a lower clinical severity of MIS‐C in paediatric age during this wave.

In conclusion, as MIS‐C can occur several weeks after SARS‐CoV‐2 infection, further studies are advocated to describe the incidence and severity of MIS‐C in children during the ongoing Omicron wave.

CONFLICT OF INTEREST

The authors declare no conflicts of interest.