Though overall US flu activity is declining, a second wave of influenza B is continuing to sicken people, especially in northeastern states, serving as a reminder that those infections can be just as severe as those caused by influenza A, the Centers for Disease Control and Prevention (CDC) said in its latest flu update.
The CDC said that, over the past several weeks, influenza B edged out the 2009 H1N1 virus's spot as the dominant flu strain, underscoring the importance of a recent study led by its researchers that compared hospitalization outcomes in adults infected by either the A or B strains. The group published its findings in an early online edition of Clinical Infectious Diseases.
The findings challenge a commonly held view that influenza B infections are milder than influenza A in adults. Influenza B is thought to be more deadly to children.
US flu markers show overall decline
In a May 2 surveillance update, the CDC said most flu markers continue to show declining activity, but for the one that tracks clinic visits for flulike illness, two of its regions—both encompassing northeastern states—are still above their specific baselines. Nationally, however, the level has been below the baseline for 7 weeks in a row.
Two more pediatric flu deaths were reported to the CDC, one caused by H3N2 and the other by an unsubtyped influenza A virus, raising the season's total so far to 91. The level of overall deaths from flu and pneumonia rose slightly from 6.3% to 6.8%, which was a bit above the seasonal baseline but still below the epidemic threshold.
Texas was the only state that reported low flulike illness activity, while the rest reported only minimal levels or not enough data to calculate a level. Four states reported widespread geographic flu spread: Connecticut, Massachusetts, New York, and Rhode Island. That number was down from six reported the previous week.
Influenza B has made up the largest portion of circulating flu viruses over the past month, with H3N2 proportions increasing as well, the CDC said.
Flu A and B severity similar for adults
In a May 2 statement, the CDC said the late-season influenza B spike highlights the importance of the Clinical Infectious Diseases study by its researchers and their state health department partners, a head-to-head severity comparison between influenza A and influenza B in hospitalized adults.
Based on data from the CDC's Emerging Infections Program, a network of 10 state health departments that conducts a wide range of surveillance and research, the researchers looked at clinical characteristics and severe disease outcomes in hospitalized adults over eight flu seasons, from 2005-2006 to 2012-2013.
The analysis included 21,186 influenza A hospitalizations and 3,579 linked to influenza B. Influenza A viruses predominated over all of the flu seasons, especially during the 2009 H1N1 pandemic.
No significant differences were seen in the proportion of hospitalizations with an intensive care unit (ICU) admission by virus type. Length of hospital stay was similar for the two virus types, as were the proportion of deaths.
The team concluded that when considering treatment options, clinicians shouldn't think of influenza B as less severe than influenza A. The CDC said the findings support its antiviral recommendations, which say that the type of flu infection shouldn't guide treatment decisions.
It added that second waves of influenza B activity are not unusual and that about 70% of the recent influenza B viruses match the B component of trivalent flu vaccines recommended for the current flu season. The quadrivalent vaccine contains strains from both influenza B lineages.
Flu in Canada, Europe
Elsewhere, Canadian health officials said on May 2 that influenza B is circulating in many of the country's regions, but with overall flu activity at expected levels for this time of year. The pattern is consistent with late-season influenza B circulation, according to the latest surveillance update from the Public Health Agency of Canada (PHAC).
The agency added that influenza B is having a greater impact on seniors and young people ages 5 to 19, compared with the 2009 H1N1 virus that predominated earlier in the season.
In Europe, influenza A is still dominant, according to a May 2 update from the European Centre for Disease Prevention and Control (ECDC). It said overall flu activity and circulation is declining, with low-intensity activity reported by 24 countries and local or sporadic activity reported for 16.
No countries reported increasing flu trends, and stable or decreasing patterns were reported for 24 countries, according to the ECDC.
May 2 CDC FluView report
May 2 CDC flu situation update
May 2 Clin Infect Dis abstract
May 2 CDC statement on influenza B study
May 2 PHAC FluWatch report
May 2 ECDC weekly influenza update