Climate factors influence seasonal influenza activity in Bangkok, Thailand

rainfall with respect to the monthly number of confirmed influenza virus infection. Average monthly temperature was typically highest from April to May (29–33˚C) and lowest in January (23–28˚C) (Fig 2A). January was also the least humid month (65–73%), while the period from September to October experienced the highest relative humidity (76–82%) and coincided with the rainy season in Bangkok (Fig 2B). Increases in the relative humidity concurred with increases in confirmed influenza A(H3N2) virus. Although the amount of rainfall varied throughout the study period, most rainfall occurred in September (10–40 cm3) (Fig 2C). However, the amount of rainfall did not correlate with the fluctuations of any influenza virus. When we evaluated the influence of climate factors in the months with and without substantial influenza activity, we found that only relative humidity significantly correlated with influenza virus (S1 Table). Table 1. Influenza virus-positive samples and monthly average meteorological factors. Positives (%) Mean (SD)b All influenza virus 6,678 (21.6)a 18.93 (10.20) A(H1N1)pdm09 2,282 (34.2)c 5.8 (7.19) A/H3N2 3,075 (46.0)c 9.13 (8.11) B 1,321 (19.8)c 4 (4.26) Meteorological factors Temperature (˚C) - 28.73 (1.71) Relative humidity (%) - 73.32 (4.96) Rainfall (cm3) - 9.11 (9.46) a All influenza virus-positive samples divided by all ILI samples submitted. b Mean and standard deviation (SD) were calculated from the monthly percentage of influenza positives relative to all monthly ILI samples.

c The number of specific influenza virus divided by all influenza virus-positive samples. https://doi.org/10.1371/journal.pone.0239729.t001 Fig 1. Monthly distribution of influenza activity from January 2010 to December 2018. Bar graph represents the proportion of ILI samples tested positive for seasonal influenza A(H1N1) (purple), influenza A(H3N2) (yellow), and influenza B (blue) virus each month. https://doi.org/10.1371/journal.pone.0239729.g001 PLOS ONE Climate factors influence seasonal influenza activity in Bangkok, Thailand PLOS ONE | https://doi.org/10.1371/journal.pone.0239729 September 29, 2020 5 / 15