Impact of COVID-19 public health interventions on influenza incidence in Thailand

The global outbreak of the 2019 novel coronavirus disease (COVID-19), as of 1 June 2020, has surpassed 6 million infections, and has resulted in over 370,000 deaths in the first five months of 2020. Thailand confirmed the first COVID-19 incidence outside of China in a traveler arriving from Wuhan on 13 January 2020 and in March, two initial domestic infection clusters among spectators at a boxing stadium and in a group of barhoppers occurred in the country’s capital city, Bangkok. The Thai government subsequently implemented incremental public health measures to mitigate COVID-19 transmission, which comprised mandatory quarantining of international travelers, keeping social distance, wearing a face mask, washing hands, measuring body temperature, closing entertainment venues and halls, restricting travel, school closures, and imposing nighttime curfews (Figure 1 panel A). As of mid-May, nearly 330,000 COVID-19 tests conducted throughout Thailand have identified approximately 3,000 infections, of which roughly half were in the capital [1].

Figure 1.

Timeline of key events and public health interventions imposed after the detection of COVID-19 in Bangkok (panel A).The observed incidence of influenza in Bangkok during the first 18 weeks of 2020 compared to the previous year (panel B). The number of influenza-like illness (shaded area, right scale), and the total number of laboratory-confirmed influenza virus-positive samples (bars) and positivity percentage (dashed line, left scale).

Prior to the COVID-19 pandemic, annual incidence of respiratory illness was mostly comprised of influenza and respiratory syncytial virus infection. Although not associated with person-to-person transmission in close-contact settings, infections by enteric viruses such as rotavirus and norovirus were also often reported in close-contact settings such as schools and social gatherings. To examine the effects of social and public health measures on the observed incidence of viral infections routinely surveyed by our laboratory, we analyzed several respiratory (influenza virus and respiratory syncytial virus) and gastroenteric (rotavirus and norovirus) virus infection trends by comparing the first 18 weeks between 2020 and the corresponding period in 2019. These ongoing studies used laboratory data of tested clinical samples from a large hospital in Bangkok as a sentinel.

There was an already marked decline in the laboratory-confirmed influenza cases and positivity in 2020, beginning in week 8, compared to the previous year (Figure 1 panel B). The total number of influenza-like illness in 2020 was significantly less than in 2019 (p < 0.05) as compared by t-test analysis. Linear regression revealed influenza trends significantly differed between 2020 compared to 2019 (ILI, −6.773 vs −4.279 per week; positivity, −2.527 vs 1.099 per week (p < 0.05)).Although the respiratory syncytial virus (RSV), human norovirus (HNoV), and rotavirus A (RVA) infections have shown decreasing trends, these viral infections generally declined during the hot summer months of March through May (Figure 2) [2–4]. Conversely, the 2020 data from the Thai Ministry of Public Health for dengue were similar to the previous year and did not show a significant change even though COVID-19 interventions were implemented [5]. Nevertheless, reduced societal activities such as daycare and school closures, avoidance of travel and social gatherings, and more time spent at home may have in part contributed to the observed reduction in influenza incidence. In addition, the reduction of influenza cases and deaths from 2019 (156,108 cases, 11 deaths) to 2020 (99,529 cases, 4 deaths) was observed during the first 18 weeks according to the national surveillance reports [6,7].

Figure 2.

The observed incidence of other viral infections in the first 18 weeks of 2020 compared with the corresponding period in 2019. Influenza-like illness and virus-confirmed samples for respiratory syncytial virus (panel A), norovirus (panel B), and rotavirus (panel C) are shown. Incidence of dengue fever (panel D) is shown as total cases and percentage.

Therefore, the advent of COVID-19 may have also inadvertently modified human behavior (through handwashing awareness, wearing masks, and social distancing) and contributed to the decreased transmission of influenza, and likely so in the foreseeable future if these behaviors become normalized. Continued stringent social and public health measures may effectively diminish the possible modes of influenza transmission.

Disclosure statement

No potential conflict of interest was reported by the authors.