ATLANTA This years flu shot is doing a startlingly dismal job of protecting senior citizens from the harshest strain this season, proving only 9 percent effective, the government said Thursday.
Health officials dont know why this is so, but it helps explain why so many older people have been hospitalized with the flu this year.
Flu vaccine tends to protect younger people better than older ones and never works as well as other kinds of vaccines. But experts say the preliminary results for seniors are disappointing and highlight the urgent need for a better vaccine.
Overall, the vaccines effectiveness is a moderate 56 percent, which means those who got a shot have a 56 percent lower chance of winding up at the doctor with the flu. That is nearly as good as other flu seasons, the Centers for Disease Control and Prevention said Thursday.
For those 65 and older, it offers far less protection. It is 27 percent effective against the three strains in the vaccine, the lowest in about a decade but not far below from whats expected.
But the vaccine did a particularly poor job of protecting older people against the toughest flu bug, which is causing more than three-quarters of the illnesses this year. CDC officials say its not clear why.
Vaccinations are recommended for anyone older than 6 months, and health officials stress some vaccine protection is better than none at all. While its likely that older people who were vaccinated still are getting sick, many of them may be getting less-severe symptoms.
Year in and year out, the vaccine is the best protection we have, and vaccinations are still recommended for senior citizens, said CDC flu expert Dr. Joseph Bresee.
To be sure, the preliminary data for seniors is less than definitive. It is based on fewer than 300 people scattered among five states.
But it no doubt will surprise many people that the effectiveness is that low, said Michael Osterholm, a University of Minnesota infectious disease expert who has tried to draw attention to the need for a more-effective flu vaccine.
Older people have weaker immune systems that dont respond as well to flu shots and are more vulnerable to the flu and its complications, including pneumonia.
Health officials at a meeting Thursday said they dont know why this years vaccine did so poorly in that age group. One theory, as yet unproven, is that seniors immune systems were accustomed to strains from the last two years and had more trouble switching gears to handle this years different, harsh strain.
Among infectious diseases, flu is considered one of the nations leading killers. On average, about 24,000 Americans die each flu season, according to the CDC.
This flu season started in early December, a month earlier than usual, and peaked by the end of year. Hospitalization rates for people 65 and older have been some of the highest in a decade, at 146 per 100,000 people.
Flu viruses tend to mutate more quickly than others, so a new vaccine is formulated each year to target the three strains expected to be the major threats. But that involves guesswork.
Because of these challenges, scientists tend to set a lower bar for flu vaccine. While childhood vaccines against diseases like measles are expected to be 90 or 95 percent effective, a flu vaccine thats 60 to 70 percent effective in the United States is considered pretty good. By that standard, this years vaccine is OK.
For seniors, a flu vaccine is considered pretty good if its in the 30 to 40 percent range, said Dr. Arnold Monto, a University of Michigan flu expert.
The CDC estimates are based on about 2,700 people who got sick in December and January. The researchers traced back to see who had gotten flu shots and who hadnt. An earlier, smaller study put the vaccines overall effectiveness at 62 percent, but other factors that might influence that figure werent taken into account.
The CDCs Bresee said theres a danger in providing preliminary results because it may result in people doubting or skipping flu shots. But the data were released to warn older people who got shots that they may still get sick and shouldnt ignore any serious flu-like symptoms, he said.
The new data highlights an evolution in how experts are evaluating flu vaccine effectiveness. For years, it was believed that if the viruses in the vaccine matched the ones spreading around the country, then the vaccine would be effective.
This years shot was a good match to the bugs going around this winter, including the harsher H3N2 that tends to make people more sick. Yet the season proved to be a moderately severe one, with many illnesses occurring in people whod been vaccinated.
A slate of new vaccines will be available next summer, including some that protect against as many of four strains of flu and some that can be made more quickly. But experts say its not clear whether they will be any more effective.