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Influenza vaccines on target, but only half effective

With at least 5 weeks left in season, CDC medical officer says patients who have not been vaccinated yet should get vaccinated

If it seems like you’ve been seeing an uptick in patients who are ill with influenza despite being vaccinated, it’s not your imagination. This season’s Northern Hemisphere vaccines are on target, yet only offer 48% protection against medically attended influenza, according to CDC officials in a February 16 Community Outreach and Communication Activity conference call and webinar.

Among samples U.S. laboratories sent to CDC since October 1, 100% of the influenza A (H1N1)pdm09 viruses and B/Yamagata-lineage viruses, 96.6% of the A(H3N2), and 90.9% of the B/Victoria lineage viruses were antigenically similar to components of the 2016–17 vaccines. Yet estimates for medically attended influenza midway through the season suggest the vaccines’ effectiveness to be just 43% for A(H3N2), the season’s most widely circulating virus. Effectiveness is estimated to be 73% for influenza B viruses.

The vaccines’ mediocre effectiveness is not surprising, according to Angela Campbell, MD, MPH, FAAP, FPIDS, FIDSA, medical officer in the Influenza Division’s Epidemiology and Prevention Branch at the National Center for Immunization and Respiratory Diseases. “Even when there is a good match, these vaccines are less effective against H3 [viruses].”

That shouldn’t be a deterrent to getting vaccinated, Campbell added. “Influenza vaccines are effective at reducing the risk of a person getting the flu and having to go to the doctor by about half. We still have [at least] 5 weeks to go this season, so patients who have not been vaccinated yet should get vaccinated.”

Estimates from the National Immunization Survey—Flu, Behavioral Risk Factor Surveillance System, and National Internet Flu Survey suggest that 37.3% of children and 40.6% of adults have been vaccinated against influenza this season thus far.

As could be expected, those older than 65 years have had the highest rate of influenza-related hospitalizations at roughly 115 per 100,000 people. Yet this season has a twist, said Alicia Budd, MPH, an epidemiologist in the Influenza Division’s Epidemiology and Prevention Branch at the National Center for Immunization and Respiratory Diseases. “What is unusual is that the 50–64 [years] age range have had more hospitalizations than the 0–4 [years] age group. The older group were hit harder early in the season.”

As of February 10, there were 25 hospitalizations per 100,000 people in the 50–64 years age group, compared to 15 hospitalizations per 100,000 people in the 0–4 years age group.

The ratios may change, said Budd. “Now the younger age group is picking up [in hospitalizations], so we we’ll have to wait until the end of the season to see if the two groups converge.”

Budd noted that influenza activity in the United States began to rise early last December, remained high through mid-January, and is expected to continue for several more weeks. For the week ending February 4, 2017, 23 states reported high rates of influenza-like illness (ILI) activity, and 10 states reported moderate ILI activity.  

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