Transmission of Respiratory Syncytial Virus Among Children Under 5 Years in Households of Rural Communities, the Philippines

Otomaru, Hirono; Kamigaki, Taro; Tamaki, Raita; Okamoto, Michiko; Alday, Portia Parian; Tan, Alvin Gue; Manalo, Joanna Ina; Segubre-Mercado, Edelwisa; Inobaya, Marianette Tawat; Tallo, Veronica; Lupisan, Socorro; Oshitani, Hitoshi

Using the dates of onset between confirmed RSV-ARI cases in the same household, the mean serial interval was estimated to be 3.2 days; in previous studies, it was estimated to be 5.6 days [11] and 7 days [12] . The 95% CI for the estimated mean serial interval in this study did not cover 1 and 2 days, which were the most frequently observed serial intervals of dates of onset between confirmed RSV-ARI cases in the same household. This means that cases with a serial interval of 1 or 2 days might not have gotten the infection from the household index case and that the real source might actually be the same source as the household index case. In this study, we assumed that 2 or more RSV-ARI cases in the same household had the same source only when they had the same date of onset, although there is no consensus among researchers on this [13] . If RSV-ARI cases with a serial interval of 1 and 2 days had the same source of infection, the actual serial interval is likely to be longer. Further studies are necessary to define the serial interval that should be considered as a common source of infection [27] . In addition, a serial interval in the same household or other close contact settings tends to be shorter than that in community settings because more extensive exposure can lead to a shorter serial interval [28] . Although different RSV subgroups may show a different serial interval, we could not analyze subgroup-specific serial intervals. In this study, we also estimated R 0 for different times (2014 and 2015) and places (Kawayan and Caibiran). We found that R 0 varied between times and places. In particular, for RSV-B in Caibiran in 2015, R 0 was high, and significantly large outbreaks were observed in 2015. We often see different patterns of RSV epidemics, including the total size [29] . These differences may be due to different factors such as the level of herd immunity for each RSV subgroup. Studies have also reported that the emergence of a new genotype within the same subgroup caused a larger epidemic [30] . Studies on RSV transmission dynamics are still limited, and epidemiological parameters such as serial intervals and R 0 have been obtained so far from limited data. In this study, it was not possible to analyze the factors affecting RSV transmission dynamics, because we conducted the study for only 2 years in 2 geographic locations. Considering the significant variations of R 0 we observed in this study, more data from different epidemics should be obtained to estimate epidemiological parameters accurately, which is necessary to establish more effective interventions, including vaccination strategies.