How well each season’s flu vaccine works to prevent illness varies from season to season, depending on the match between the flu viruses the vaccine is designed to protect against and the flu viruses that are circulating when the flu season arrives. Recent news coverage about a study done in Australia related to the effectiveness of their flu vaccine has sparked rumors and some misinformation, so we talked to Dr. Jeff Duchin, King County Health Officer and Chief of Communicable Disease Epidemiology and Immunization, to clarify what this study means here in the United States.

The study from Australia found that their flu vaccine was not very effective in preventing influenza  A H3N2, the predominant strain circulating in the Southern Hemisphere during their recent flu season. Are these findings predictive of what we should expect in the U.S.?

JD: No, not necessarily. It is not unusual for vaccine effectiveness study results to vary from country to country, for multiple reasons. For example, there can be different results based on differences in the level of immunity against the circulating and related influenza viruses that already exists in the population of that country. There may be differences in the age distribution of the population and differences in the number of people with underlying health conditions, such as diabetes or asthma, and weakened immune systems that can make difference in effectiveness rates. The extent of vaccine coverage in a community during the previous flu season can also make a difference.  Finally, the size of the study can also influence the reliability of the results. The Australian study was relatively small, making results less precise.

Another factor is the relative proportion of infections in the community caused by the two types of influenza A (H3N2 versus H1N1) and the two types of influenza B that can circulate each season. Vaccine effectiveness against influenza A H1N1 and influenza B strains is usually much better than against H3N2.

The study stated that Australia’s flu vaccine effectiveness for influenza A H3N2 was only 10%. Does that mean that it will be the same in the U.S.?

Currently, the predominant strain in the U.S. is also influenza A H3N2, which is the same strain that circulated last season and that circulated in the recent Southern Hemisphere flu season. But that doesn’t necessarily mean that the U.S. flu vaccine effectiveness for H3N2 will be the same as in Australia. Last year in the U.S., vaccine effectiveness for influenza A H3N2 was 34%, and there have been no significant mutations detected in this strain since the last season.

Influenza B is also circulating, though at lower levels. We can’t be sure of how much illness each of the circulating types influenza viruses (A H3N2, A H1N1, two types of influenza B) will contribute until the season is over. Vaccine effectiveness against influenza A H1N1 and influenza B strains is usually much better than against H3N2, about 50-60% overall.

So it sounds like the flu vaccine is still worth getting, even if we don’t know exactly how well it will match the flu viruses this season?

Absolutely. The bottom line is that even though we cannot predict which strains will predominate in the US this season, it is likely that influenza vaccination will provide meaningful protection against severe illness, hospitalization and death. During the 2012-13 flu season, vaccine effectiveness was similar to what is expected this season based on currently available information. During that season, flu vaccination prevented: 5.6 million illnesses, 2.7 million medical visits, 61,500 hospitalizations and 1,820 deaths.

Influenza vaccine is especially important for pregnant women to protect both mother and her newborn, and for other people at increased risk for serious influenza infections, including complications, hospitalization and death.

See the link below for who is at increased risk and should get vaccinated: https://www.cdc.gov/flu/about/disease/high_risk.htm

Originally posted on December 7, 2017.