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Current flu season hitting younger people especially hard

Friday February 21, 2014
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This influenza season has been particularly hard on younger and middle-age adults, according to a CDC report.

People ages 18 to 64 represented 61% of all hospitalizations from influenza, up from the previous three seasons, when this age group represented only about 35% of all such hospitalizations, according to the report, which was published in the Feb. 21 issue of the Morbidity and Mortality Weekly Report.

Influenza deaths followed the same pattern, with more deaths than usual occurring in this younger age group.

An accompanying report showed that influenza vaccination offered substantial protection against the flu this season, reducing a vaccinated person’s risk of having to go to the doctor for flu illness by about 60% across all ages.

“Flu hospitalizations and deaths in people younger and middle-aged adults is a sad and difficult reminder that flu can be serious for anyone, not just the very young and old, and that everyone should be vaccinated,” CDC Director Tom Frieden, MD, MPH, said in a news release. “The good news is that this season’s vaccine is doing its job, protecting people across all age groups.”

U.S. flu surveillance data suggests that flu activity is likely to continue for a number of weeks, especially in places where activity started later in the season. Influenza rates are decreasing in some states with earlier increases in flu activity. Other states continue to experience high levels of flu activity or further increases in activity.

While flu is responsible for serious illness and death every season, the people who are most affected can vary by season and by the predominant influenza virus. The currently circulating H1N1 virus emerged in 2009 to trigger a pandemic, which was notable for high rates of hospitalization and death in younger and middle-aged people.

While H1N1 viruses have continued to circulate since the pandemic, this is the first season since the pandemic in which they have been predominant in the U.S. The virus again is causing severe illness in younger- and middle-aged people.

The data

Approximately 61% of flu hospitalizations so far this season have occurred among people ages 18 to 64. Last season, when influenza A (H3N2) viruses were the predominant circulating viruses, people ages 18 to 64 accounted for only 35% of hospitalizations. During the pandemic season of 2009-10, people ages 18 to 64 accounted for about 56% of hospitalizations.

Hospitalization rates also have been affected. While rates are still highest among people ages 65 and older (50.9 per 100,000), people ages 50 to 64 have the second-highest hospitalization rate (38.7 per 100,000), followed by children ages 0 to 4 (35.9 per 100,000). During the pandemic, people ages 50 to 64 also had the second-highest hospitalization rate. (Hospitalization rates are cumulative and thus will continue to increase this season.)

Influenza deaths this season are following a pattern a similar to the pandemic. People ages 25 to 64 have accounted for about 60% of flu deaths this season, compared with 18%, 30% and 47% for the three previous seasons, respectively. During 2009-10, people ages 25 to 64 accounted for an estimated 63% of deaths.

“Younger people may feel that influenza is not a threat to them, but this season underscores that flu can be a serious disease for anyone,” Frieden said. “It’s important that everyone get vaccinated. It’s also important to remember that some people who get vaccinated may still get sick, and we need to use our second line of defense against flu: antiviral drugs to treat flu illness. People at high risk of complications should seek treatment if they get a flu-like illness. Their doctors may prescribe antiviral drugs if it looks like they have influenza.”

People at high risk for flu complications include pregnant women; people with asthma, diabetes or heart disease; people who are morbidly obese; and people older than 65 or children younger than 5, but especially those younger than 2.

Vaccine effectiveness

In a flu vaccine effectiveness study, CDC researchers looked at data from 2,319 children and adults enrolled in the U.S. Influenza Vaccine Effectiveness Network from Dec. 2, 2013, to Jan. 23, 2014. They found that flu vaccine reduced the risk of having to go to the doctor for flu illness by an estimated 61% across all ages.

The study also looked at vaccine effectiveness by age group and found that the vaccine provided similar levels of protection against influenza infection across all ages. Vaccine effectiveness point estimates against influenza A and B viruses by age group ranged from 52% for people 65 and older to 67% for children 6 months to 17 years.

Protection against the predominant H1N1 virus was even slightly better for older people: Vaccine effectiveness against H1N1 was estimated to be 56% in people 65 and older and 62% in people ages 50 to 64. All findings were statistically significant.

The interim vaccine effectiveness estimates this season are comparable to results from studies during other seasons when the viruses in the vaccine have been well-matched with circulating influenza viruses, according to the CDC, and are similar to interim estimates from Canada for 2013-14 that were published recently.

While flu vaccine can vary in how well it works, vaccination offers the best protection available against influenza infection. The CDC recommends that everyone 6 months and older get an annual flu vaccine.

“We are committed to the development of better flu vaccines, but existing flu vaccines are the best preventive tool available now,” Frieden said. “This season vaccinated people were substantially better off than people who did not get vaccinated. The season is still ongoing. If you haven’t yet, you should still get vaccinated.”

Flu prevalence report: www.cdc.gov/mmwr/preview/mmwrhtml/mm6307a3.htm

Vaccine effectiveness report: www.cdc.gov/mmwr/preview/mmwrhtml/mm6307a1.htm


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