US Flu Activity Falls Below Baseline After Longest Flu Season in a Decade

Although the current influenza season in the United States has been milder than 2017-2018, the US Centers for Disease Control and Prevention (CDC) says a second wave of the virus made this season the longest in a decade.

The good news is that during influenza season week 16 ending on April 20, 2019, the proportion of outpatient visits for influenza-like illness (ILI) fell to 2.1%, the first time in 21 weeks that the rate was not at or above the national baseline of 2.2%. According to the CDC’s weekly FluView report, influenza activity continues to decrease following a second wave of flu illnesses that began in February when influenza A(H3N2) viruses started to predominate. Prior to this year, the longest recent flu season was in 2014-2015 and lasted 20 weeks, and although this season was longer, last season was more severe and led to nearly 80,000 deaths.

Through April 20^th, the CDC estimates that there have been as many as 42.4 million flu illnesses, 630,000 hospitalizations, and 59,500 flu-related deaths this season. In week 16 the CDC also reported 5 influenza-associated pediatric deaths, bringing this season’s total to 96. The 2017-2018 flu season saw 186 pediatric flu deaths in the US, the most since the 2009-2010 flu season, which saw 288 pediatric flu deaths.

In more pediatric flu news, a new study published in Open Forum Infectious Diseases by investigators in Austria has found that repetitive influenza infections in consecutive flu seasons occurred quite frequently in children and adolescents, predominantly in young children. The study builds on prior research about how influenza infections in the first decade of life may help build long-lasting immune memory and protection in subsequent years after an infection against similar influenza virus types or subtypes.

The study included a retrospective analysis of 2308 laboratory-confirmed influenza cases in children and adolescents during the flu seasons from 2014-2015 to 2017-2018. Investigators found that in the 2015-2016 flu season, nearly 12% of patients also had an influenza infection during the previous season; in 2016-2017, more than 14% had at least 1 infection during the prior 2 flu seasons; and in 2017-2018, more than 18% had 1 or more infections during the 3 previous seasons. Of the patients, 29 had 3 or 4 infections during these seasons, and 38 children had 2 influenza episodes within the same season. Most of the repeat infections occurred in children aged 3 to 8 years.

The findings, according to the investigators, both support and challenge findings from prior studies, suggesting that subsequent infections with other influenza A or B virus subtypes in childhood result in a similar long-lasting immunological memory and result in at least partial protection from clinical infection. “During the observation period we found a number of patients with 3 or even 4 infections. Those children had either 1 or 2 influenza A virus infections and 1 or 2 influenza B virus infections, but no child had more than 2 influenza A or 2 influenza B virus infections, respectively,” the investigators wrote. “We also observed consecutive infections within the same season. Of interest, none of these children had received a vaccination in the previous 3 seasons and the vast majority never in lifetime.”

“According to our data it appears that consecutive and subsequent infections in children mainly occur in the first decade of life with a peak between 3 and 8 years, at a time when long-lasting immune memory is most likely to be developed,” the research team concluded.

“Therefore, the question arises on how often we can get influenza in [a] lifetime specifically when we experienced infections during the first decade.”