I’ve often heard people decline the vaccine because they once got flulike symptoms after the injection, or they came down with the flu despite it.

Facts: 1) The vaccine does not, and cannot, cause the flu. Either they were infected before they were immunized or before immunity took hold, or the fever and aches they experienced were side effects of the vaccination, not the flu itself. 2) As currently formulated, the vaccine is far from perfect. In some years, it has been as little as 20 percent effective in preventing the flu, although it may still lessen the severity of the illness and likelihood of complications.

The biggest problem with flu vaccine is the need to determine months in advance of flu season which of the virus variants to include. The flu virus is a moving target, highly subject to mutations; even the way the vaccine is made — often involving growing the virus variants in eggs — can cause them to mutate.

If the variants included in the vaccine differ from those that are ultimately responsible for seasonal outbreaks, protection is likely to be greatly reduced, which is what happened in the 2004-05 and 2014-15 flu seasons, when vaccine effectiveness was only 10 percent and 19 percent, respectively. Last season’s vaccine was about 36 percent effective.

There is also a difference in the protection afforded by the same vaccine given to people in different age categories. For example, the vaccine used against flu in 2012-13 had an overall effectiveness of 49 percent, but was only 11 percent effective for people 65 and older. A higher-potency vaccine is now available for older men and women.

There is now a concerted effort to create a new “universal” flu vaccine that would protect against all manner of variants and not require an annual shot. The goal, Dr. Osterholm said, is “a vaccine that can handle many new changes in the virus and that needs to be given only once every five or 10 years.”

The basic research needed to develop such a vaccine could cost $1 billion a year for the next five to seven years, he estimated. However, Congress allocated a mere $100 million for the coming fiscal year.