Large-scale sampling for covid-19 had an after effect, as more samples were also analyzed for RS, influenza and calicivirus (winter vomiting disease) during the 2021/2022 season. Much more cases of RS and calicivirus have been reported to the Swedish Public Health Agency than in previous seasons. This is not least compared to the 2020/2021 winter season when the number of reported cases was unusually low, as evidenced by the agency’s latest collections.

Large-scale sampling has resulted in more cases outside health care being confirmed and reported during this season for all of these viruses. But what also distinguishes this season from previous years is that the pattern and timing of the spread of infection appears to be different, says Anasara Karnahan, a researcher and epidemiologist at the Swedish Public Health Agency.

The pandemic continued to affect the spread of common seasonal viruses. Among other things, this meant an unusually severe and early epidemic of RS that infected many young children in the fall of 2021. The spread of influenza and winter vomiting also showed unusual patterns with two transmission peaks.

The low prevalence of RS, calicivirus and influenza during the past winter season 2020/2021 is likely to have increased susceptibility in the population, which may have contributed to more intense periodic spread of infection in 2021/2022, Annasara Carnahan says.

RS . virus

The RS virus season started already early in September 2021, and then led to a severe epidemic among children under the age of five. In total, nearly 6,800 laboratory-verified cases of RS have been reported among children aged 0-4 years, which is an 89 percent increase compared to the last intensive season 2018/2019. A total of 244 patients in intensive care were treated with RSV and an increased healthcare burden was seen during peak weeks. The intense season is likely due in large part to the fact that many groups of young children were not previously infected, as RS did not spread to the same extent during the first year of the epidemic. Therefore, many young children were more susceptible to infection, which also contributed to the emergence of an epidemic earlier than usual. Similar epidemics of RS have also been seen in other countries.

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The calicivirus (winter vomiting disease) season did not occur in 2020/2021, but the 2021/2022 season was more intense as a large number of samples were taken and the Swedish Public Health Agency reported. Additionally, for the first time ever, two peaks were observed during one and the same season. In the past, peak was the norm. In total, nearly 6,000 cases of the callus virus have been reported, which is a 61 percent increase compared to the average in the past three seasons before the pandemic. It is possible that the absence of the previous season contributed to the population becoming more susceptible to infection and leading to a more severe season.


The 2021/2022 influenza season was unusual with two waves with an average period of low activity. The prevalence of infection is thought to be lower than normal with significant regional differences. The general assessment is that the first wave of the season in December 2021 was of moderate intensity, while the second wave in spring 2022 was less intense. In total, just over 13,000 cases were reported during the season, almost all of whom are estimated to have had influenza A (H3). During the season, 110 patients were reported in influenza intensive care nationally, which is lower than the average of 311 patients in the 2016/2017 to 2019/2020 seasons.

The Swedish Public Health Agency’s compilation of vaccination coverage shows that nearly 70 percent of people aged 65 or over in Sweden were vaccinated against seasonal influenza during the 2021/22 season, which is five percentage points higher than the previous season.

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