Flu activity declined in the United States again last week, but is still high for this time of the year, and new research based on data from Canada this season reveals an age-group of children hit harder by the 2009 H1N1 strain because they weren't alive during the pandemic years.
Researchers and public health officials closely monitor attack rates across different age-groups, not only to gauge the burden of the disease but also to watch for immunity gaps that might influence spread patterns and response to vaccines. A team from Canada published its findings on disproportionate level of cases in children younger than 10 yesterday in Eurosurveillance.
Fewer states report widespread flu
According to the weekly snapshot from the US Centers for Disease Control and Prevention (CDC), 20 states reported widespread flu, down from 33 and Puerto Rico the week before. The percentage of clinic visits for flulike illness is still above the national baseline, as it has been for 20 weeks in a row, but declined from 3.1% to 2.8%.
The percentage of respiratory specimens that tested positive for flu was 15.1%, down from 19% the previous week. In the middle of February, H3N2 nudged out 2009 H1N1 as the dominant strain, and in another unusual development this season, few influenza B viruses—which usually rise late in the season—have been reported. H3N2 is now outpacing H1N1 76.8% to 23.2% among influenza A–positive strains.
One flu marker that rose slightly last week was the hospitalization rate, which increased from 56.4 to 59.9 per 100,000 population. The highest rates are in adults ages 65 and older (195.9 per 100,000 population).
Four more pediatric flu deaths were reported, pushing the season's total 86. One was from 2009 H1N1, one from H3N2, and two from unsubtyped influenza A viruses. Overall deaths from pneumonia and flu fell below the epidemic threshold, to 7.1%.
Study highlights vulnerable group of kids
The 2009 H1N1 virus has been the dominant flu strain in the Northern Hemisphere this season, and surveillance systems in Canada reported that children were more affected by the virus than other age-group have been. For the study, researchers from British Columbia, Quebec, Ontario, Alberta, and Manitoba examined age distribution of the 2009 H1N1 cases this season compared with past flu seasons, and they compared unvaccinated people against test-negative controls.
Findings across the different flu seasons showed that children younger than 10 made up a greater proportion of outpatient visits this season, especially kids ages 5 to 9 years old, who made up 14% of cases, twice that for earlier 2009 H1N1–dominant years in the 2013-14 and 2015-16 flu seasons.
Danuta Skowronski, MD, the study's first author, who is with the British Columbia Centre for Disease Control and the University of British Columbia in Vancouver, said the paper, as expected, shows that school-age children (ages 5 to 19) were overrepresented in earlier H3N2 epidemics. However, younger school-aged kids (ages 5 to 9) hadn't been overrepresented in earlier 2009 H1N1 epidemics.
She said for the first time this season, the younger school-aged group is fully made up of kids who weren't yet born during the 2009 H1N1 pandemic, missing out on high infection rates that conferred pandemic-related immunity. Skowronski notes, in contrast, that preteens and teens— who have greater social contacts than their younger peers and were alive during the pandemic years—have been underrepresented across all 2009 H1N1 dominant seasons since then.
The authors wrote that, besides kids their own age, younger school-age kids have contacts with siblings and parents that tend to be longer duration, creating richer contact networks that may have amplified spread beyond their peers.
So far, it's not clear if flu attack rates this season were enough to reduce the susceptibility in this "moving cohort" of children who weren't alive during the 2009 H1N1 pandemic, the authors note. "If not, that susceptibility may next extend to include those older than 10 years who have, on average, the greatest number of effective contacts, a potentially precarious combination for future A(H1N1)pdm09 epidemics."
Skowronski said immunologic cohort effects related to major priming events such as pandemics need to be followed in this way to anticipate age-related vulnerability during ensuing flu seasons.
Apr 12 CDC FluView report
Apr 12 CDC flu situation report
Apr 11 Eurosurveill report