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THIS YEAR’S FLU season is looking a lot milder than 2014-15. So far, flu-related outpatient visits — one measure of a season’s severity — has remained under 2.5% of the U.S. population, compared to last year when they peaked in late December at around 6%. The reason: in 2014-2015, most of the flu vaccines did not include an especially virulent strain that appeared just weeks after the strains were selected for that year’s vaccine.
Flu vaccines in general work best in healthy young adults and older children. A higher dose shot is recommended for those ages 65 and older because an individual’s immune response to vaccines declines with age, producing up to 75% fewer antibodies than in younger adults, according to one study.
The “Fluzone High-Dose” vaccine has four times the standard dose of antigen, which stimulates the body’s production of flu-fighting antibodies. Although one study showed the high-dose vaccine to be almost 25% more effective in preventing flu in adults over 65, the CDC has not yet published a preference for any flu vaccine for this age group.
The CDC recommends a flu shot for everyone six months of age and older. The vaccine is often available by the end of the summer, and people are advised to get it by October or soon afterwards, because it can take up to two weeks to produce sufficient flu-fighting antibodies. Even when the vaccine fails to target a prevalent strain, it protects against the other two or three which might be circulating and usually comes close enough to lessen the severity of infection in those who get the missing strain.
The biggest challenge in creating the flu vaccine each year is prediction. To allow enough time to produce more than 170 million doses for the U.S. market, the WHO and CDC must decide in February of the preceding year which strains of both the A and B influenza virus will prevail the following fall. The problem is this “production lag,” Dr. Arnold Mondo, a flu expert at the University of Michigan told the L.A. Times.
When prediction is close, the flu vaccine can reach 70% efficiency. The vaccine for 2014-2015 was estimated to be around 50% effective, according to Dr. Joseph Bresee of the CDC. Flu vaccines usually include two strains of influenza A — H1N1 and H3N2 —along with one or two strains of influenza B. In 2014, the strain of H3N2 chosen for the Northern Hemisphere was “Texas,” while the strain “Switzerland” was chosen for the Southern Hemisphere. But Switzerland showed up in the north in March, too late to add it to that year’s vaccine. (Unusually, in 2009-2010, during the H1N1 pandemic (originally referred to as swine flu), people were advised to get an additional vaccination against H1N1 along with the flu vaccine of the season.)
By September of 2014, the prevalence of this mutated Switzerland strain had increased exponentially. And by the time the Northern flu season was in full swing that fall, some 80% of the H3N2 viruses had mutated or “drifted” to the Switzerland strain. The prevalence of H3N2 strains often leads to more severe flu illnesses, hospitalizations and deaths. In addition, the 2014-2015 flu season lasted 20 weeks compared to the average of 13 weeks in the previous 13 flu seasons.
In that 2014-2015 season, 322 out of 100,000 people were hospitalized for flu compared to the previous average of 183 out of 100,000 people. Even in 2007-2008 when the strain of H3N2 was also a “drift variant,” the effectiveness was higher than it was for 2014-2015.
Flu vaccines are available in several forms, with some more highly recommended for specific ages. The traditional vaccine uses “inactivated” viruses — using virus particles grown in the lab and then killed. It is given “intramuscularly” and can include three or four strains — called “trivalent” with three strains, or “quadrivalent” with four. For people who dislike shots, the intradermal vaccine uses a needle 90% smaller than typical vaccination needles, which barely penetrates the surface of the skin and uses 40% less antigen than regular shots.
Nasal spray vaccines, approved for ages 2 through 49 and all quadrivalent, can be more effective because they contain live virus that has been weakened or attenuated. Because these viruses are alive, they can replicate, albeit harmlessly, in the body to create an immune response. Last comes the jet injector, recommended for ages 18 to 64, which uses a high-pressure, narrow stream of fluid instead of a needle to deliver a trivalent vaccine. Developed in the 1960s, it has been used previously in mass inoculations, for example, against smallpox and for military personnel.
The flu vaccine can have side-effects including mild fever and soreness at the site of the injection. A very rare but crippling side-effect is Guillain-Barre Syndrome, which can cause paralysis lasting months or longer.
For the future, flu experts are working on a universal vaccine made using live virus that is collected in the upper respiratory system of from people infected with the flu and then attenuated to prevent it from infecting the lungs. This vaccine would use a newly discovered class of “universal” antibodies found to be as effective as those that are strain-specific and would protect against all strains of flu, even when the virus mutates, according to Matthew Miller, senior author of the study at McMaster’s University in Ontario.
— Mary Carpenter