This influenza season was particularly hard on younger- and middle-age adults, the Centers for Disease Control and Prevention reported in today’s Morbidity and Mortality Weekly Report. People age 18-64 represented 61 percent of all hospitalizations from influenza—up from the previous three seasons when this age group represented only about 35 percent of all such hospitalizations. Influenza deaths followed the same pattern; more deaths than usual occurred in this younger age group.
A second report in this week’s MMWR 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 percent 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,” says CDC director Tom Frieden, MD, MPH. “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. Some states that saw earlier increases in flu activity are now seeing decreases. Other states are still seeing high levels of flu activity or continued 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 they have been predominant in the U.S. Once again, the virus is causing severe illness in younger- and middle-aged people.
Approximately 61 percent of flu hospitalizations so far this season have occurred among persons aged 18-64 years. Last season, when influenza A (H3N2) viruses were the predominant circulating viruses, people 18 to 64 years accounted for only 35 percent of hospitalizations. During the pandemic season of 2009-2010, people 18 to 64 years old accounted for about 56 percent of hospitalizations. Hospitalization rates have also been affected. While rates are still highest among people 65 and older (50.9 per 100,000), people 50 to 64 years now have the second-highest hospitalization rate (38.7 per 100,000), followed by children 0-4 years old (35.9 per 100,000). During the pandemic, people 50 to 64 years also had the second-highest hospitalization rate. Note that 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 25 years to 64 years of age have accounted for about 60 percent of flu deaths this season compared with 18 percent, 30 percent, and 47 percent for the three previous seasons, respectively. During 2009-2010, people 25 years to 64 years accounted for an estimated 63 percent 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," says Frieden. "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 years, but especially those younger than 2 years. A full list of high risk factors and antiviral treatment guidance is available on the CDC website. More information about flu vaccine and how well it works also is available.
In the flu vaccine effectiveness (VE) study, CDC looked at data from 2,319 children and adults enrolled in the U.S. Influenza Vaccine Effectiveness (Flu VE) Network from December 2, 2013 to January 23, 2014. They found that flu vaccine reduced the risk of having to go to the doctor for flu illness by an estimated 61 percent across all ages. The study also looked at VE by age group and found that the vaccine provided similar levels of protection against influenza infection across all ages. VE point estimates against influenza A and B viruses by age group ranged from 52 percent for people 65 and older to 67 percent for children 6 months to 17 years. Protection against the predominant H1N1 virus was even slightly better for older people; VE against H1N1 was estimated to be 56 percent in people 65 and older and 62 percent in people 50 to 64 years of age. All findings were statistically significant.
The interim VE 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 and are similar to interim estimates from Canada for 2013-14 published recently.
While flu vaccine can vary in how well it works, vaccination offers the best protection currently available against influenza infection. 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. 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," says Frieden.