So why isn't the nasal spray working well now? Researchers don't know for sure, but it could have to do with the strains of influenza that have been circulating in recent years. For the past several flu seasons, the dominant flu strain was H1N1, the same strain that caused a flu pandemic in 2009. It's possible that the nasal spray isn't very effective at protecting against the H1N1 strain, Adalja said.
Other research supports this idea. A study published in the journal Pediatrics in January found that the nasal spray worked just as well as the flu shot at protecting children from infection with the influenza strain H3N2 and the influenza B strain during recent flu seasons. But kids who received the nasal spray were three times more likely to become infected with H1N1 than kids who received the flu shot, the study found.
This lower effectiveness of the nasal spray "was unexpected," the researchers said. The results may have to do with the particular strain of H1N1 flu virus included in the nasal spray vaccine. Studies have found that this flu strain has a mutation that lowers its ability to cause infection in animals, which could also reduce its ability to provoke a protective immune response.
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In addition, this H1N1 strain may be more susceptible to degradation at high temperatures, studies have found. Even though vaccines are refrigerated, they still can be exposed to high temperatures at some points during the shipping process, such as when they are unloaded from trucks or unpacked from their boxes, according to a 2015 presentation from MedImmune, the company that makes the FluMist nasal spray. Thus, the greater susceptibility of this virus to heat could reduce vaccine effectiveness, according to the presentation, and the researchers proposed tweaking the vaccine to make it more heat-stable.
AstraZeneca, which owns MedImmune, said in a statement today (June 23) that other studies of FluMist conducted in the 2015-to-2016 flu season have found that the nasal spray was up to 58 percent effective at preventing flu, and that the company still plans to distribute the nasal spray in other countries.
The effectiveness of the flu vaccine can vary from season to season depending on a number of factors, including which flu viruses are circulating and how well the viruses in the flu vaccine match those that are circulating, the CDC said.
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It's important to note that the new recommendation against using the flu nasal spray is only for the upcoming 2016 to 2017 flu season, Adalja said. After that, the CDC will need to review new data to see if it will keep or change this recommendation.
The CDC recommends flu vaccines for everyone ages 6 months and older. The agency said it will work with vaccine manufacturers to ensure that there is enough flu vaccine to meet demand this fall.
Original article on Live Science.
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