Local territory-wide monthly laboratory data on the PCR-based detection of norovirus and rotavirus, the two leading causes of viral gastroenteritis (7), are publicly available since January 2013 from the Centre for Health Protection of Hong Kong (equivalent in function to other national public health agencies such as the China CDC) (8). Laboratory data for less common diarrheagenic viruses including sapovirus, astrovirus and enteric adenovirus were available from May 2017 onwards. From January 2013 through September 2021, a total of 104,187 stool specimens collected from sporadic and outbreak cases of acute gastroenteritis were tested. The median number of specimens tested each month was 1,008 (interquartile range [IQR]: 912—1,114) and 872 (IQR: 784—990) before and during the COVID-19 pandemic, respectively. Although an average of 13.5% less stool specimens were tested during the pandemic (P<0.01; Mann-Whitney U test), the reduced sample sizes were still of sufficient power to detect at least one positive specimen under a virus prevalence as low as 0.5% (namely 1 in 200) at a 95% confidence level.
Monthly positive rate of the five common viral causes of acute gastroenteritis is shown in the Figure. During winter 2019/20, the positive rate of rotavirus dropped abruptly from peak by 70% in February 2020, shortly after the initial global spread of COVID-19 and remained at a much lower level of 0.1%—0.6% through September 2020 compared with a median of 5.4% (IQR: 2.8%—13.4%) during the same period in the past seven years. The winter 2019/20 season of rotavirus ended about two months earlier than usual. Considering that the observed lower positive rate may be confounded by the declining trend in recent years with the availability of rotavirus vaccines, we further examined data on norovirus in which no effective antivirals or vaccines are presently available. Likewise, norovirus positive rate dropped sharply from peak by 56% in February 2020 and remained at a much lower level of 0.3%—1.5% through September 2020 compared with a median of 6.4% (IQR: 5.2%—10.0%) during the same period in the past seven years. The winter 2019/20 season of norovirus ended almost three months earlier than usual. We have previously shown that norovirus exhibited a bimodal seasonality in Hong Kong with two peaks every year, one in winter and another in summer/autumn (9). Of note, the expected norovirus peak in summer/autumn 2020 was not observed in this study. For sapovirus, astrovirus and enteric adenovirus, these viruses became hardly detectable since the start of the COVID-19 pandemic, showing a positive rate of persistently below 1% throughout 2020 and 2021.
In winter 2020/21, a typical seasonal peak of norovirus with a positive rate of 10.3% was observed in January 2021, and the rate was comparable to the median of 14.4% in the past seven winter seasons. Likewise, a typical seasonal peak of rotavirus with a positive rate of 4.8% was observed in January 2021, and the rate was highly comparable to that of 5.3% and 6.2% in the past two winter seasons, albeit on a progressively decreasing trend in recent years. These data indicated active circulation of norovirus and rotavirus in the community during winter 2020/21 while strict non-pharmaceutical interventions for COVID-19, including work-from-home arrangement for civil servants, mandatory mask-wearing outside homes, school closure and prohibition on group gatherings of more than four persons in public places, were in effect in the city.