The arrival of spring has brought higher temperatures and sunny weather across much of the US. Meanwhile, flu season lingers, recently reaching the mark of longest in the past decade. According to the CDC’s Weekly U.S. Influenza Surveillance Report for the week ending April 13, seasonal influenza activity “remains elevated” above the national baseline for the 2018-19 flu season. STAT reports that there have been 21 weeks of elevated flu activity in 2018-19, one of the longest flu seasons since the government began tracking flu season duration. Below, we provide a brief overview of CDC’s seasonal influenza surveillance efforts, including how it classifies the start and duration of flu season, followed by a discussion of some of the factors that may have contributed to this year’s increased flu season duration.

US Flu Surveillance

The CDC uses several measures to describe the state of flu seasons in the US. The CDC determines elevated flu activity regionally and nationally by comparing current data against baseline values for influenza-like illnesses (ILIs). Sentinel surveillance sites across the US report data on the percentage of outpatient visits due to ILIs. Regional and national baseline values are set at 2 standard deviations above the 3-year mean percentage outside of flu season. Geographic spread of flu transmission is categorized by the presence of flu activity in each state. “No Activity” means no lab-confirmed cases and no increase in ILIs. “Sporadic” corresponds to lab-confirmed cases but no increase in ILIs. “Local,” “Regional,” and “Widespread” are defined by flu outbreaks or the combination of lab-confirmed cases and increased ILIs—”Local” is a single region in the state, “Regional” is more than one but fewer than half of the regions in a state, and “Widespread” is more than half the regions in a state. Other efforts include virologic surveillance, which characterizes the circulating strains of influenza virus, and hospitalization and mortality data, which help characterize the severity of influenza disease in a given season.

The CDC officially determines the beginning of flu season based on “sustained elevated activity...observed across key indicators for a number of weeks,” but it typically reports the season duration in terms of the number of weeks that national ILI outpatient visits remained above baseline. Interestingly, these may not necessarily be the same dates.

2018-19: An Exceptionally Long Flu Season

The CDC reported the beginning of the 2018-19 flu season as Week 50 (ending December 15), but ILIs “had hovered at or slightly above the national baseline...for the previous 3 weeks.” The epidemic is now poised to stretch into May. This week (Week 15), the CDC reported that ILIs accounted for 2.4% of outpatient visits at sentinel sites across the US, although cases are on the decline. During this reporting period, 11 states reported widespread flu activity, with another 20 reporting regional activity and 17 states reporting local activity.

One potential reason for the extended duration of this year’s flu season is the unexpected rise in incidence of a second strain of influenza in late winter. Influenza type A (H3N2) usurped influenza type A(H1N1) as the most commonly reported strain around late February/early March. During Week 15, the CDC reported that more than 70% of reported specimens were positive for the H3N2 strain (236 out of 333 subtyped specimens). The occurrence of separate waves of dominant circulating strains has led some experts to comment that it is essentially like battling two flu seasons in one.

Fortunately, flu season duration does not necessarily correlate with severity. The 2017-18 flu season, for example, was particularly bad in the US, resulting in an estimated 48.8 million cases and nearly 1 million hospitalizations and 80,000 deaths (including 183 reported pediatric deaths). Last year was driven largely by H3N2 throughout, which likely factored heavily into its severity. As we have noted previously, H3N2 has been found to be more virulent than other circulating influenza strains, which has contributed to increased influenza-associated hospitalizations in the past, and flu vaccines are typically considered less protective against H3N2 than other strains. Fortunately, just because this year’s flu season was longer, it was not as severe as what we saw in 2017-18.

Conclusion

Cases of both strains (H3N2 and H1N1) have already peaked and are on the decline, suggesting that the 2018-19 flu season may soon be coming to a close, although elevated activity could continue into May. Official CDC estimates will be reported after the flu season officially winds down, so it will take some time before we can more accurately compare 2018-19 to other recent flu seasons. However, if the current total of 91 pediatric deaths is any indication, it is likely on par with most recent seasons and far less severe than last year. Regardless, seasonal flu continues to spread in many parts of the US, and we must remain vigilant, including recommended protective actions such as proper hand hygiene and cough etiquette.

Photo: Electron microscopic image of influenza virus particles.

Photo courtesy of CDC/Dr. F.A. Murphy

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