One theory, as yet unproven, is that older people’s immune systems were accustomed to strains from the last two years and had more trouble switching gears to handle this year’s different, harsh strain.
The preliminary data for senior citizens is less than definitive. It is based on fewer than 300 people scattered among five states.
But it will no doubt 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 vaccine.
Among infectious diseases, flu is considered one of the nation’s 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 strains expected to be the major threats. CDC officials have said that in formulating this year’s vaccine, scientists accurately anticipated the strains that are circulating this season.
Because of the guesswork involved, 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 that’s 60 to 70 percent effective in the U.S. is considered pretty good. By that standard, this year’s vaccine is OK.
For senior citizens, a flu vaccine is considered pretty good if it’s in the 30 to 40 percent range, said Dr. Arnold Monto, a University of Michigan flu expert.
A high-dose version of the flu shot was recently made available for those 65 and older, but the new study was too small to show whether that has made a difference.