Jun 26, 2008 (CIDRAP News) – Eighty-three children in the United States died of influenza during the past flu season, the Centers for Disease Control and Prevention (CDC) said today in a report confirming previous indications that the season was a relatively rough one.
The 2007-08 season was worse than the previous two but similar to 2004-05 in terms of the percentage of deaths due to pneumonia and influenza, children's hospitalizations, and doctor visits, the CDC says in the Jun 27 issue of Morbidity and Mortality Weekly Report.
The report also confirms previous evidence that the season's vaccine did not match up very well with many of the circulating flu viruses and that resistance to the antiviral drug oseltamivir increased.
The CDC received reports of 83 children (younger than 18) in 33 states who died of flu-related causes during the season. That compares with 68 children in the 2006-07 season and 35 children in 2005-06, according to previous CDC reports.
The mean age of the children who died was 6.4 years, and the median was 5.0. Of the 63 children older than 6 months whose vaccination status was known, 58 (92%) had not received flu shots.
The overall hospitalization rate for children (0-17 years) was 1.54 per 10,000, according to surveillance data from the Emerging Infections Program. For young children—0 to 4 years—the rate was 4.03 per 10,000. By comparison, the rates for young children in the previous three seasons ranged from 2.5 to 3.8 per 10,000.
Deaths attributed to pneumonia and flu exceeded the CDC's statistical epidemic threshold for 19 weeks in a row and peaked at 9.1% of all deaths in the week of Mar 9-15, the report says. In the previous three flu seasons, the number of weeks this number hovered above the epidemic threshold ranged from 1 to 11, and the peak percentage varied from 7.7% to 8.9%.
The proportion of visits to sentinel healthcare providers for treatment of flu-like illnesses surpassed national baseline levels for 13 weeks, from late December to late March, the CDC says. That number peaked at 6.0% the week of Feb 17-23. During the three previous seasons, the peak percentages of visits that were flu-related ranged from 3.2% to 5.4%.
Compatibility of vaccine and viruses
The report offers final figures on the match between the three viral strains used in the vaccine and those that made people sick. The biggest problem was with influenza B: of 350 virus isolates that were antigenically characterized by the CDC, only 6 matched well with the strain used in the vaccine.
For influenza A/H1N1, 66% of the 407 viruses analyzed by the CDC were similar to the vaccine strain. But for influenza A/H3N2—the predominant strain for the season—only 23% of 404 viruses that were characterized matched well with the vaccine strain.
The season also was marked by an increase in resistance to antiviral drugs. Specifically, 10.9% (111 of 1,020) of the H1N1 viruses analyzed by the CDC were found to be resistant to oseltamivir, versus only 0.7% of H1N1 viruses tested last season. However, none of the H3N2 or type B viruses were resistant to the drug, and all tested viruses were susceptible to zanamivir (the other antiviral in the class known as neuraminidase inhibitors).
Most H3N2 viruses continued to be resistant to the older antiviral drugs, amantadine and rimantidine. The CDC continues to recommend oseltamivir and zanamivir, but not the older drugs, for treating and preventing flu.
In the article, the CDC predicts that the flu vaccine supply will be "abundant" for next season, which will be the first year in which vaccination is specifically recommended for all children aged 5 to 18 years.
CDC. Influenza activity—United States and worldwide, 2007-08 season. MMWR 2008 Jun 27;57(25):692-97 [Full text]
Apr 17 CIDRAP News story: CDC: Imperfecct flu vaccine was of help in tough season