That 23% figure is a measure known as “vaccine effectiveness,” and it’s certainly on the low end of the spectrum. In the decade since experts began calculating a “VE” for flu vaccines, it has ranged from a low of 10% to a high of 60%.
But even at 10%, a vaccine could prevent about 13,000 flu-related hospitalizations among senior citizens in the U.S., the CDC researchers and their colleagues wrote in Thursday's edition of the Morbidity and Mortality Weekly Report.
If patients agreed to participate, researchers used a genetic test to see whether they indeed had the flu (and if so, which strain). They also asked volunteers whether they had gotten a flu shot or flu mist vaccine at least two weeks before they became sick.
As of Jan. 2, 2,321 children and adults had joined the study. The researchers found that 49% of the people who tested positive for the flu had been vaccinated, along with a slightly higher percentage – 56% – of those who tested negative.
After the researchers controlled for factors like age, gender and baseline health, they determined that the vaccine reduced the risk of needing to see a doctor for treatment of a flu-like illness by 23% overall.
Normally, researchers don’t make calculations about vaccine effectiveness until later in the flu season. But since flu activity is already widespread in 46 states (including all five states in the Flu VE Network), the researchers had enough data to make a first pass at their calculations. As the flu season wears on and more volunteers join the study, the VE may rise (or, in the case of adults, the lower VEs may become statistically significant).
Dr. Tom Frieden, the director of the CDC, has warned that this year's flu season could cause more sickness and death than usual because H3N2 flu strains are dominating. Making matters worse, the North American vaccine is a good match for only about one-third of the H3N2 viruses that are circulating here. The rest are of a type that didn't show up here until March – after the World Health Organization had decided which strains the vaccine should target.
In years when the vaccine is a good match for circulating viruses, the vaccine effectiveness has been in the 50% to 60% range.