In Austria comprehensive lockdown measures for prevention of further SARS-CoV-2 spread were adopted on March 16th 2020 (calendar week 12). In Figure 1 the weekly numbers of influenza, RSV, hMPV and RHIV cases detected by the sentinel system are shown for the season 2019/20, before and after the lockdown. For all these respiratory viruses a significant decrease was observed within one week after the lockdown. To assess whether the development of cases of influenza viruses, RSV, hMPV and RHIV in 2019/20 was different to previous seasons, a comparative analysis between the respiratory virus circulation pattern of this and the previous 5 seasons was performed. The data of the respiratory infections over the years are presented in detail in Figure 2 and in Figure S1.
The differences between the seasons were calculated for the individual viruses. For comparison of influenza viruses (Figure 2a) those seasons between 2014/15 to 2018/19 were chosen, which were comparable with respect to influenza type distribution. For these selected seasons the average number of cases for each week weighted by the number of tested patients was computed and then the cumulative number of cases for the duration of activity was obtained. The total number of cases from week of lockdown until end of activity was computed for season 2019/20 and the same computation was performed for the corresponding period of the comparator seasons to obtain the expected number of cases. The hypothesis of equality of the number of observed and expected cases was tested by chi² tests with 1 degree of freedom assuming a Poisson distribution. The influenza season 2019/2020 already peaked between calendar weeks 04 and 06/2020. Based on data of previous influenza seasons in Austria an end of the influenza season would have been expected at around week 15 to 16/2020. By implementing the lockdown measures, the influenza season 2019/20 was ended earlier than expected and its course differed significantly from that of previous seasons (p<0.001) (Table 2).
For comparison of seasonal RSV and hMPV (Figure 2b, c), seasons were chosen with respect to the shape of the seasonal activity distribution until the week corresponding to the week of lockdown 2019/20. Also for RSV a sharp decrease was observed after the nationwide lockdown and, although this year’s RSV activity was generally low, reduction in the circulation of RSV detected after week 12 was significantly different compared to previous seasons (p<0.001). Also, the lockdown measures led to a significant decrease in the number of hMPV detections (p<0.001) (Table 2), which was especially impressive, as the hMPV season 2019/20 started with a very high hMPV activity. For both viruses circulation remained low until week 28. Detailed analyses on the reduction of cumulative cases are provided in Supplementary material Figure S2.
To compare the non-seasonal RHIV circulation between different years, the cumulative number of cases between calendar week 12 (start of lock down) and week 22 was computed for all seasons (Figure 2d). The 5 seasons 2014/15 to 2018/19 were compared to season 2019/20 by chi² test with 5 degrees of freedom. Also for RHIV a statistically significant decrease of cases was observed after the lockdown (p<0.001) (Table 2).