RSV activity in Japan
Current respiratory syncytial virus (RSV) activity in Japan — based on WHO FluNet positivity-rate surveillance, set against the parallel flu and COVID-19 trajectories.
Current situation: RSV
In week 14 of 2026, activity of respiratory syncytial virus (RSV) in Japan is low. The trend — derived from clinical surveillance — is stable. Activity has barely shifted over the past few weeks.
The classification is based on the WHO FluNet aggregate of sentinel laboratory data from Japan's National Institute of Infectious Diseases (NIID), which includes RSV positivity indicators. Seasonally, infection waves in Japan typically peak between December and February; activity is usually markedly lower in spring and summer. How severe a given season becomes depends on the circulating virus variant and the population's immune status, among other factors.
Data sources and methodology
The current picture for Japan is built on WHO FluNet, the World Health Organization's global platform for influenza surveillance. the National Institute of Infectious Diseases (NIID) reports weekly sentinel laboratory results to FluNet, which we translate into a consultation-equivalent signal.
WHO FluNet
FluNet is the WHO's weekly global influenza surveillance database. National influenza centres and reference laboratories — for Japan this is the National Institute of Infectious Diseases (NIID) — submit the number of respiratory specimens tested and the number positive for influenza and, where available, RSV. Using FluNet ensures a consistent, internationally comparable data source for countries without a European-style sentinel consultation system.
Positivity-based signal
Unlike ECDC ERVISS, FluNet does not carry outpatient consultation rates. Our headline signal is therefore derived from the weekly lab positivity rate (percent of tested specimens that are positive), multiplied by a pathogen-specific scaling factor — 80 for influenza and 50 for RSV — to approximate a consultation-equivalent incidence. COVID-19 is not available via FluNet and is therefore not shown for these countries.
Why this source
FluNet is the only weekly, comparable dataset covering many non-European countries. Positivity is relatively insensitive to changes in overall testing volume, which makes it a reasonable proxy for underlying transmission pressure even when the denominator fluctuates. We apply a SENTINEL-only filter where appropriate to reduce noise from non-sentinel reporting streams.
Qualitative classification
The “low”, “moderate” and “high” categories follow seasonal reference values and epidemiological thresholds calibrated to match our classifications for other countries. The positivity × scaling-factor product is mapped to the same consultation-equivalent scale we use elsewhere so results stay comparable across regions. Data refreshes weekly when WHO publishes the latest FluNet update.
Frequently asked questions
When is RSV season in Japan?
Japan's RSV season has historically peaked in autumn, with recent years showing shifts toward summer and early-autumn peaks — particularly since 2021. NIID tracks weekly RSV activity through its paediatric sentinel network. The season is typically more prolonged than the flu peak and disproportionately affects infants and very young children.
How is RSV monitored in Japan?
NIID coordinates RSV surveillance through roughly 3,000 paediatric sentinel sites reporting weekly case counts, supplemented by virological testing at prefectural reference laboratories. Weekly IASR bulletins describe RSV activity at national and regional levels. These indicators are also shared with WHO FluNet under the Western Pacific regional reporting framework.
Who is most at risk from RSV in Japan?
In Japan as elsewhere, the highest-risk groups are infants — especially those under six months and those born preterm — and older adults with underlying cardiopulmonary conditions. Paediatric hospital admissions for RSV bronchiolitis cluster sharply during peak weeks. Infant monoclonal antibody prophylaxis and adult RSV vaccination are available for qualifying risk groups.
Why is RSV timing in Japan so different from Europe?
Japan's climate, indoor-contact patterns, and paediatric-care structures produce an RSV curve that can peak earlier in the calendar year than European curves. Recent years have seen summer peaks in Japan that contrast sharply with central European winter peaks. Because both regions feed comparable surveillance data into international systems, these differences are directly observable in WHO's Western Pacific and ECDC reports.
How does RSV differ from flu in Japanese data?
NIID tracks RSV and influenza separately through different sentinel networks — RSV through paediatric sites, influenza through a broader primary-care network. RSV signals are dominated by very young children and typically lead or lag the flu curve depending on the year. Clinical overlap between the two makes laboratory confirmation the reliable basis for distinguishing them.
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