The COVID-19 pandemic claimed millions of lives and interrupted global society—but along the way it also changed the transmission landscapes of respiratory pathogens, including influenza (flu) virus and respiratory syncytial virus (RSV).

Our research is pioneering an investigation into flu and RSV shifts by measuring "weekly hospitalizations" on a cross-country scale rather than, as done previously, focusing on infection cases in individual countries. By understanding how the seasonality has changed, this work can inform the delivery of vaccines and antivirals to reduce the disease and economic burdens connected to these pathogens arriving off season.

Irregular influenza and RSV trends occurred worldwide throughout the pandemic. In 2020, South Africa's winter influenza epidemic seemed to have stopped entirely. In 2021, the United States grew sick as RSV surged off season. In 2022, Chile reported an early start to its influenza season, and by the end of the year, the World Health Organization (WHO) and the European Centre for Disease Prevention and Control issued a joint statement warning about the early kick-off of the influenza season epidemic and intensified RSV activity in Europe. At the time COVID-19 was still a public health emergency of international concern (PHEIC), and many public health experts worried about a collision of these diseases, coining them a twindemic or tripledemic.

4.3 Weeks

Influenza hospitalization peaks have been occurring 4.3 weeks earlier in five temperate countries

Yet the shift remained even as COVID-19 slowed. An out-of-season RSV epidemic occurred in early 2023 in Beijing, and as of April 2024 influenza season has started early in Australia.

Respiratory infectious diseases remain a challenge to health-care systems. Stephen Powis, medical director of the England's National Health Service, warned that pressure on England hospitals in the 2023–2024 winter is "not letting up," with thousands more beds occupied and three times as many influenza patients as last year. Any additional or atypical spikes in demand could throw the recovery from COVID-19 further off course.

Individuals could become susceptible to viruses if they are unaware of risks present in their schools, workplaces, and communities—leading to extra strain on economies. Because vaccine-induced immunity decreases over time, it is important for people to protect themselves in the months before epidemic peaks. To ensure public health and social productivity, communication strategies should be tailored for optimal timing of influenza and RSV vaccinations.

Patterns in Hospitalizations Pre- Versus Post-Pandemic

Our research uncovered substantial changes when focusing on six countries—Australia, Brazil, England, France, Turkey, and the United States. Those countries kept comprehensive reports of influenza and RSV hospitalizations between 2017 and 2023.

The influenza hospitalizations have peaked in the five temperate countries, excluding Brazil, 4.3 weeks earlier on average since the 2019–2020 season. Peaks in RSV hospitalizations were more erratic. They arrived 1.3 weeks earlier on average in the five temperate countries after the pandemic, but during the 2020–2021 season a notable 11.2 weeks average delay in RSV hospitalization offset subsequent differences.

Overall, the impact of COVID-19 is likely diminishing over time. In the 2021–2022 season, the average peaks for influenza and RSV among all countries occurred 3.8 and 5.9 weeks earlier, respectively, relative to 3.1 and 2.9 weeks earlier in the 2022–2023 season. However, on comparing pre- and post-pandemic numbers across these temperate countries, the differences in average hospitalization rates are not significant.

Flu Seasons Peak Earlier in England

Post-pandemic cases peaked in December rather than January or February

Looking closer at the countries from the Northern Hemisphere, higher influenza and RSV activity were consistently observed during the winter months before the pandemic. The post-pandemic hospitalizations, however, looked different in terms of distribution and reporting pattern.

The fluctuation in RSV circulations was striking during the 2020–2021 season. France, for example, experienced a delayed RSV epidemic in March 2021, and England an out-of-season peak in July 2021, mirroring the timing typically seen in the Southern Hemisphere. Influenza patterns were harder to parse in the Northern Hemisphere countries during this early period of the COVID pandemic. During the 2020–2021 season, influenza hospitalizations decreased, as observed in England, but countries such as Turkey and the United States temporarily discontinued their flu surveillance systems due to COVID spikes.

Looking at the Southern Hemisphere, Australia recorded changes in the timing of peak influenza hospitalizations despite the suspension of surveillance system in 2020 and 2021.

Brazil saw various shifts in both influenza and RSV activities, and this inconclusive finding indicates a greater diversity in seasonality across tropical regions, as seen in previous research.

Global Seasonality Shifts

Flu hospitalizations peak earlier in both Northern and Southern Hemispheres, and RSV shows a similar pattern in Turkey

Policy Implications

Offsetting the normal course of respiratory pathogens matters in part because every February and September, the WHO recommends viral strains for inclusion in influenza vaccine composition ahead of Northern and Southern Hemisphere winter seasons, respectively. Monitoring the post-pandemic changes, the WHO may need to consider whether convening consultations earlier is necessary.

"We're expecting an early flu season and flu vaccines have just become available," said Australian Mary-Anne Thomas, the Victorian health minister, at the beginning of April 2024. That is a good example of how the Ministry of Health closely monitors disease dynamics, adjusting vaccination planning and timeline accordingly.

For pharmaceutical industries, the production pipeline of vaccines and antiviral medications could need to be scaled up well in advance of the usual timelines to accommodate rising demand during atypical disease seasons. Some could even argue that resource allocation is more crucial than increasing production.

Will These Nations Return to Normality Next Season?

In short, no one knows.

Several pandemic factors have potentially influenced the observed shifts. The first concerns contact patterns. Influenza and RSV notification rates fell sharply during the COVID era of school closures and travel bans, but lifting lockdowns and other public health social measures (PHSMs) could result in resurging and out-of-season admissions.

What we learned from COVID-19 is the uncertain nature of pathogens and human behavior

The second factor relates to immunity, as the U.S. Centers for Disease Control and Prevention stated in 2022: "Reduced population immunity, particularly among young children who may never have had flu exposure or been vaccinated, could bring about a robust return of flu." Declines in vaccine uptake in certain regions, combined with changes in vaccination attitudes and vaccine fatigue, further complicate that issue.

Building on these factors, scientists have tried to understand the effects of COVID-19 PHSMs on the transmissibility of influenza and RSV, offering predictions for the future. Nevertheless, what we learned from COVID-19 is the uncertain nature of pathogens and human behavior. Despite WHO's declaration of the end of COVID's PHEIC in May 2023, long-term seasonality and the impact of COVID-19 remain uncertain—let alone whether normal influenza and RSV trends will be seen again and when.

Timing is everything. When transitioning from the acute phase of COVID-19 to co-circulation, it becomes essential to implement year-round monitoring for multiple pathogens with overlapping symptoms profiles. Reporting to global surveillance databases such as FluNet, FluID, and GISAID is also important. Doing so can support health governance at global and country scales, ensuring timely planning and delivery of health protection programs.

CDC Director Robert Redfield explains influenza-like illness (ILI) surveillance programs at the White House. — CDC Director Robert Redfield explains influenza-like illness (ILI) surveillance programs at the White House, in Washington, DC, on April 17, 2020. REUTERS/Leah Millis

ACKNOWLEDGMENTS: The authors thank Associate Professor Rosalind Eggo at the London School of Hygiene and Tropical Medicine for her expert guidance and thank Associate Professor Thomas Cowling and Assistant Professor Simon Procter for making this possible.

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Where COVID Has Shifted Flu and RSV Seasons

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