What Caused RSV to Roar Back During COVID?

Reopening schools likely played a critical role in the rebound of respiratory syncytial virus (RSV) infections since late 2020, researchers found.

Full reopening of schools across multiple countries was associated with a 23 times higher risk in the rebound of RSV infections, albeit with a wide confidence interval (HR 23.29, 95% CI 1.09-495.84), reported You Li, PhD, of Nanjing Medical University in China, and colleagues.

Conversely, warmer weather, such as a 5°C increase in temperature, was tied to a lower risk of RSV rebound (HR 0.63, 95% 0.40-0.99), they wrote in the Journal of Infectious Diseases.

Li's group noted that countries that implemented non-pharmaceutical inventions at the beginning of the pandemic likely saw a reduction in the circulation of RSV in 2020. In the U.S. during the height of the pandemic, positive RSV rates dropped by 97.4% from seasonal averages even though testing rates remained fairly steady, according to the CDC and a 2021 analysis of electronic health record data.

But since then, some countries have "observed delayed out-of-season RSV rebound since late 2020," while others have not.

To disentangle the potential reasons why, the authors performed a time-to-event analysis across 18 countries, including 11 European countries, Canada, Australia, and New Zealand, that had data on RSV activity from 2019 to 2021. Exposures of interest included status of school openings, population mobility, ban on international arrivals, COVID notification rate, and meteorological factors.

RSV rebound was defined as the first onset of RSV after the expected RSV season during the COVID pandemic, the authors said. The last week of available RSV data was Sept. 8, 2021.

While all 18 countries experienced delayed RSV onset, 61% observed RSV rebound, which was delayed by 5-54 weeks based on expected RSV onset.

The main model used by Li's group found that full reopening of schools was associated with risk of RSV rebound, but the same was not true for partial reopening of schools. Interestingly, the association between school reopening and RSV rebound was non-significant in the complete model that included all exposures of interest.

When assessing the relationship between these variables, the authors found that full reopening of schools may substantially increase the risk of RSV rebound, especially with decreased temperatures, and even higher temperatures. The authors explained that for all comparisons, the reference temperature was set as 10°C (50°F), which is "the median temperature when a typical RSV season occurs in the 18 countries and reference week was week 0."

Not surprisingly, closing schools could "gradually decrease the risk" for RSV rebound, "although to a lesser extent when temperatures decrease," they wrote.

"The risk for RSV rebound also increases over time since the expected typical RSV onset, highlighting the role of the increased susceptible population," Li's group noted. "Growing susceptibility and full (re)-opening of schools could both override the counter-effect of high temperatures, which explains the out-of-season RSV epidemics during the COVID-19 pandemic."

They predicted a "surge in RSV cases" that could happen earlier than a typical RSV season.

Limitations to the data include potential confounders, such as a decline in international travel and viral interference. The authors emphasized that "we focused our analysis on the timing of RSV rebound; due to data scarcity, we were unable to evaluate how different factors could affect the magnitude or severity profile of RSV rebound."

"It will be important to continue, or in some cases re-establish, surveillance for RSV at this stage of the COVID-19 pandemic to both better understand the epidemiology of RSV transmission, as well as prepare for the burden of RSV rebound on the public health system," they wrote.

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