October 22, 2017
2 min read

Influenza pandemics more likely to emerge in spring, summer

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A recent modeling study showed that influenza pandemics are more likely to occur in spring and summer months than during the height of a seasonal influenza epidemic.

“These findings may improve pre-pandemic risk assessments and real-time situational awareness, particularly as we gain greater insight into the extent of immunity,” Lauren Ancel Meyers, professor at the University of Texas, and colleagues wrote in PLoS Computational Biology.

According to the researchers, the six most recent influenza pandemics that occurred in the Northern Hemisphere emerged in March (1918), April (1957, 2009), May (1889, 1977) and July (1968). Although the similar timing of these pandemics may be a coincidence, the researchers hypothesized that pandemic strains may not easily spread at the height of a seasonal influenza epidemic because infection with one influenza virus can provide temporary immunity to other viruses.

“On the one hand, we would expect pandemics to emerge during the flu season, when socio-environmental conditions are conducive to influenza transmission and co-infections are likely; on the other hand, those would be the months of greatest competitive interference,” they wrote. “These competing effects suggest that the risk of pandemic emergence may be greatest at the tail of the flu season, when conditions are still favorable and co-infections are possible, but competition is waning.”

Meyers and colleagues developed a model that simulated influenza transmission patterns to predict when pandemics will likely emerge. The model was based on real-world data from the 2008-2009 influenza season and the assumption that people recovering from a seasonal influenza infection had full heterosubtypic immunity for 42 days.

Image depicting influenza transmission patterns
This image depicts hypothetical influenza transmission patterns. The colors indicate regions with seasonal influenza infection (red), regions currently immune to pandemics (purple), and regions that are currently susceptible to a novel pandemic (blue). The white lines portray the global flight network.
Source: Spencer J. Fox

The researchers estimated that naturally occurring immunity reduced the probability of a pandemic emergence by 73% in winter months when immunity is at its highest. It also reduced the reproductive number of an emerging virus by 30%. Despite these findings, they warned that their study is “intended as a proof of concept,” and there is still much they cannot predict.

“We don’t know when or where the next deadly flu pandemic will arise. However, the typical flu season leaves a wake of immunity that prevents new viruses from spreading,” Meyers said in a press release. “Our study shows that this creates a narrow, predictable window for pandemic emergence in the spring and early summer, which can help public health agencies to detect and respond to new viral threats.” – by Stephanie Viguers

Disclosures: The authors report no relevant financial disclosures.