Flu vaccines are only 23% effective this season — bad news in a particularly harsh year for the virus, health officials said Thursday.Flu shots normally prevent 60% to 65% of infections serious enough for people to see a doctor.
This year's influenza vaccines are less effective than usual, however, because the viruses used to make them aren't a good match for the viruses now spreading across the country, according to the Centers for Disease Control and Prevention. Scientists select the viruses to be used in flu vaccines in the spring, in order to give manufacturers enough time to produce the vaccines. But this year, scientists noticed that influenza A (H3N2) — the dominant strain in circulation — began mutating almost immediately.
About 68% of these viruses have now "drifted" to be genetically different from the flu viruses used in vaccines, the CDC said.
The H3N2 virus is especially nasty and usually causes twice as many hospitalizations and deaths compared to other viruses, the CDC said. Even in a year with a "good match" between virus and vaccine, flu shots tend to be less effective against H3N2, preventing only about 40% of infections, said Arnold Monto, an influenza expert and professor of epidemiology at the University of Michigan School of Public Health.
"This speaks to the fact that we need better flu vaccines," Monto said.
Flu shots have always been less effective than other vaccines, such as measles. Two doses of measles vaccine prevent 99% of infections.
Researchers are trying to develop a "universal" flu vaccine that would last for many years. To do this, scientists would have to try to stimulate immunity against proteins inside a flu virus, which don't change from year to year, Monto said. Now, flu vaccines aim to produce immunity against proteins on the surface of the flu virus, which change all the time.
A universal flu vaccine is still many years away, however, Monto said.
The CDC still recommends that people get flu shots. Fewer than half of Americans have been vaccinated this year, slightly less than usual, according to the CDC.
Even a less effective vaccine can still prevent thousands of hospitalizations, especially among seniors. Young people — who often think they don't need a flu shot — can help protect seniors and other vulnerable people by getting vaccinated, Monto said. That's because vaccination reduces the amount of viruses in circulation, so there is just less flu to make people sick, Monto said.
Although the USA is about halfway through the flu season, viruses will likely continue to circulate for six weeks or so, the CDC said.
CDC officials note that flu vaccines protect against three to four different strains of flu viruses. So a flu vaccine may protect people against different strains of virus, such as influenza B, which often shows up late in the season.
The CDC is also urging patients and health providers to make greater use of antiviral drugs, such as Tamiflu and Relenza. If taken within the first 24 to 48 hours after symptoms appear, antivirals can reduce the length of time that people are sick by one or two days, Monto said. Antivirals also reduce the risk of serious complications and hospitalizations. They may offer some benefit even after 48 hours, especially in elderly people.
"They work, but they aren't widely used," Monto said