The 10 new flu-related deaths in children bring the season's total to 226, the most since 2009-10.
Plaque growth can lead to a higher risk of heart attack, stroke, and other life-threatening cardiovascular events for as long as 1 year.
Post-exertional malaise, or exercise intolerance, was seen in 36% of those with long COVID.
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Infected women were more likely to need tracheostomy and ventilation and to have heart attack, sepsis, and blood clots.
The top three conditions were anxiety, depression, and post-traumatic stress disorder.
In other developments, the WHO added XBB.1.9.1 to its monitoring list, a subvariant that is slowly rising in the United States and other countries.
The authors said findings support the importance of vaccinating women during and after pregnancy.
The 30-day death rate was 5.97% for COVID-19 and 3.75% for flu, for an excess death rate of 2.23%.
The authors called for a clearer definition of long COVID so that evidence from clinical trials can be reliably applied to evidence-based clinical management.
In the two rare cases, babies had seizures at birth, microcephaly, and developmental delays over time.
The authors say neither a human nor animal origin can be ruled out.
The ECDC said though no clear seasonal circulation pattern has emerged, COVID's impact has been higher in the fall and winter.
The relative risks in some UK ethnic groups were higher earlier in the pandemic but shifted toward White, higher-income groups in a later wave.
