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This year's flu vaccine is even more disappointing than previously reported, showing just 18 percent effectiveness against the dominant H3N2 strain of flu, according to health officials.
That's a drop from the 23 percent protection level estimated for the flu shot earlier in the season, said experts at the U.S. Centres for Disease Control and Prevention.
The situation for children was even worse. The CDC panel pegged the effectiveness of the injected vaccine for kids aged 2 to 8 to be just 15 percent.
And the nasal-spray version of the vaccine may not protect young children at all, health officials said.
"Studies can't confirm that the [nasal] vaccine has a benefit," said Dr. Joseph Bresee, chief of the Epidemiology and Prevention Branch at CDC's National Centre for Immunisation and Respiratory Diseases.
Why the poor performance?
"These low numbers, which are lower than we normally see, are because the flu viruses that are circulating have mutated to look very different than the vaccine strains," Bresee explained.
Read: The three types of flu viruses
He added that the H3N2 flu strain that is currently circulating was not factored into this season's vaccine because decisions about which strains to target were made last February. As happens each year, choices about which strains to include are made months in advance so that manufacturers have time to make enough vaccine, Bresee explained.
In fact, this new strain of H3N2 was seen for the first time in March 2014 and didn't become dominant until September, Bresee noted. "That was much too late to make a new vaccine," he said.
"This is a very unusual circumstance where a new strain develops and becomes a dominant strain so quickly and after the vaccine had already been produced," he said.
The Word Health Organisation made its recommendation for the makeup of next season's vaccine, Bresee said. And the U.S. Food and Drug Administration will decide which strains will be included in next season's flu vaccine for the United States. Bresee said the FDA usually follows the WHO recommendations.
Only somewhat effective
WHO recommended that this year's mutated H3N2 strain as well as updated versions of other strains be included in next season's vaccine, Bresee added.
Although this year's vaccine is only somewhat effective and the flu season has peaked, Bresee still recommends getting a flu shot.
"Year in and year out, flu vaccines are still the best way to protect yourself against flu," he said. "Most years, the vaccine is 50 to 70 percent effective. The vaccine doesn't protect everybody every time, but it's odds on the best way to protect yourself against flu."
Dr. Marc Siegel, a professor of medicine at NYU Langone Medical Centre in New York City, agreed that the vaccine is still the best protection against flu.
"Yes, it's still worth getting it though the season has already peaked," he said.
Children hit hard
The CDC said that the flu season isn't over yet, and there is an increasing number of B viruses circulating, which are a better match to the vaccine and could bring final vaccine effectiveness numbers up.
This year's flu season has also hit children hard, the agency said, with 86 children dying from complications of flu. For context, the CDC noted that in an average year, child deaths from flu vary from as few as 30 to as many as 170 or more.
Read: Kids who were preemies need their flu shots
The CDC currently recommends that everyone aged 6 months and older get vaccinated. Even if the flu shot isn't a perfect match to circulating strains, vaccination can still prevent some infections and reduce severe disease that can lead to hospitalisation and death, the agency said.
Other ways to treat and prevent flu from spreading include early treatment with antiviral drugs such as Tamiflu and Relenza, and washing hands frequently and covering your mouth when coughing or sneezing.
Early treatment with antiviral drugs is especially important for children aged 2 and younger, along with adults aged 65 and older, the agency said. Others for whom these vaccines are essential are people with diabetes, heart disease or breathing problems, they added.
Read more:
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Nearly half of Americans think flu shot can make you sick
Image: Flu shot from Shutterstock
