Effectiveness of flu shot Is 60% — in a good year

Bug alert: Australia recorded more flu infections this year than ever before, and the same virus is now rapidly circulating in the U.S.

Five children have died here, and around 750,000 people have come down with flu-like illnesses, according to the Centers for Disease Control and Prevention.

Health officials worry about the flu because it can lead to pneumonia and other life-threatening illnesses, and it’s impossible to predict when the next pandemic will occur.

The last one erupted in 2009 when an estimated 60.8 million people got sick in the U.S., 274,000 were hospitalized and more than 12,000 died.

How many people get the flu in any given year depends in part on the effectiveness of the vaccine. In a good year, people who get vaccinated are 60% less likely to get sick, but in most recent years, the protection has been worse.

Last year, it was 42% effective. In 2014-15, it was 19% effective. During the 2009 pandemic, it was 56% effective, but the virus spread quickly that year before the vaccine was available.

Four categories of flu make people sick—the “A” viruses H1N1 and H3N2, and the “B” viruses Victoria and Yamagata. Flu vaccines include strains of both “A” viruses and at least one “B” virus, but each can mutate, and to be effective, vaccines must match the circulating strains.

The World Health Organization meets twice a year to decide which strains the vaccines should include.

The recommendation for the Southern Hemisphere is made in September. The recommendation for the Northern Hemisphere is made in February or March. In each case, distribution of the flu vaccine begins about seven months later.

“You’re having to guess which strains circulating now will go on to dominate all of winter,” said Michael L. Jackson, an epidemiologist who investigates vaccines and infectious diseases at the Kaiser Permanente Washington Health Research Institute.

This year, manufacturers projected up to 166 million doses of injectable flu vaccine would be available for flu season, which generally begins in October or November. By mid-October, about 130 million doses had been distributed, and so far, 77% of the circulating H3N2 virus matches the vaccine.

The CDC recommends that everyone ages 6 months and older get a flu shot, even if the effectiveness is modest compared with other vaccines, such as the one for measles, which is about 97% effective.

“It’s a low-risk thing to do,” said IHPI member Emily Martin, an epidemiologist at the U-M School of Public Health. “If you can do this and cut your risk in about half that you’ll get sick, it’s not a bad gamble.”