This year’s flu shot is doing a fairly average job of protecting people against the pervasive winter virus, according to the annual interim effectiveness report released Thursday by the U.S. Centers for Disease Control and Prevention.
The agency said the shot has been 45% effective overall and 55% effective in children. The vaccine was less effective for influenza A than B and did considerably worse among 18- to 49-year-olds, where it was only 25% effective. Against one type of influenza A, H1N1, it had no proven effectiveness in younger adults. Previous seasons have ranged from 40% to 60% effective when vaccines were well-matched to circulating strains of virus. The vaccine performed particularly poorly last year with an overall effectiveness level of just 36%.
By the CDC’s definition, effectiveness does not address your personal chances of getting the flu after a shot. It looks at a population’s chance of getting sick enough to seek medical attention. So, people who got this year’s shot were 45% less likely to see doctors for flu than those who were not vaccinated, said Brendan Flannery, an epidemiologist with the CDC’s influenza division.
As it did last year, the CDC said you should still get a shot if you haven’t already. It estimated last week that so far this season at least 26 million people have gotten the flu, 250,000 have been hospitalized and 14,000 have died.
The CDC estimate was based on a study of 4,112 patients with respiratory illnesses in five states, including Pennsylvania. About half of them had been vaccinated. The vaccination rate was highest in people ages 65 and older—82% – and lowest in 9- to 17-year-olds—34%.
Stephen Gluckman, an infectious-diseases doctor at Penn Medicine and director of Penn Global Medicine, said that, even though the shot is imperfect, it’s “well worth getting because there’s so much flu. From a public health perspective, even something that’s preventing half the cases is preventing millions of cases.” Gluckman said it’s fairly late to be getting the shot now, but there’s no downside to it. He warns patients that the shot takes a while to become effective and there’s a lot of flu around. If they get sick in four or five days, it’s not the shot’s fault.
Flannery said it is a “mystery” why younger adults have not been as well protected by the shot. However, he said it was good news that the vaccine has worked well in children, who have been especially hard hit by flu this year. Some flu experts were concerned that the vaccine would not perform well because it was not a perfect match for the strain of influenza B that dominated during the early part of the season.
While a vaccine for a childhood illness like measles is 80% to 90% effective, Flannery said, no one expects numbers like that for flu, a virus that mutates frequently. Gluckman pointed out that it also takes months to produce flu vaccines. It can be tough for pharmaceutical companies to produce the right kind of vaccine in time.
This year’s flu season got off to an early start and flu activity has remained at high levels for several weeks. Reports from New Jersey and Pennsylvania Health Departments showed that flu activity remained high through last week, but there were signs that it might be abating.
The CDC said 92 children had died of influenza-related illness, the most for this point in the season since the 2009 flu pandemic. Flannery said the numbers are not complete yet, but said the majority of deaths occur in unvaccinated children.
An average flu season lasts 18 weeks. This one started during the first full week of November, so this season is in its 16th week.
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