Flu viruses are constantly changing. This perpetual transformation is why influenza vaccines have to be updated from season to season.
But this year is unusual. There will be no change in the seasonal flu vaccine. Public health officials are betting that the United States will see the same flu strains that circulated last year.
That makes it much easier to produce the vaccine. But there's also a risk that people may not bother getting vaccinated.
The three flu strains in this year's vaccine have been here before.
A California H1N1 is the same virus that caused the pandemic outbreak two years ago primarily among younger people. So far, the strain hasn't changed or faded.
The new vaccine also contains A Perth H3N2. That virus reappeared as a dominant strain last year and caused a lot of illness among the state's elderly population.
The B flu strain in this year's vaccine is Brisbane, also unchanged from last year.
"We're happy to say that the strains that are circulating now look like the vaccine strains are going to cover them well," said Dr. Carolyn Bridges, an immunization specialist at the Centers for Disease Control and Prevention.
Each year U.S. health officials pick three strains to include in the seasonal flu vaccine based on what they see circulating elsewhere in the world. It's not a precise science because flu viruses can easily and quickly change after the vaccine selections have been made. So far, there's no sign of that.
Kris Ehresmann, infectious disease division director at the Minnesota Department of Health, said the advantage to sticking with the same vaccine this year is that manufacturers already know the best way to grow the strains.
"In this case they already kind of had all that information and so they could just go right ahead and produce it," Ehresmann said. "So the fact that vaccine was shipping and becoming available in July is really great."
It also could be a little confusing.
Ehresmann, who manages the state's immunization program, said some people have asked her if the vaccine being advertised now is leftover from last year. It's not.
Still, she suspects some Minnesotans will be reluctant to get another dose of the same vaccine they had the previous year.
"I think it is going to be challenging," Ehresmann said. "When you hear it's the same strain there's sort of this feeling of complacency that, 'Oh, I don't really need to get another shot, do I?' "
Some people may not need a flu shot. But there's no way to know for sure, so public health officials are urging people to have one anyway.
Dr. Lisa Grohskopf, a medical officer in the CDC's influenza division, said some people might retain their flu protection from one year to the next. But studies show that others do not.
"It's not really possible to tell for any given individual exactly how quickly their antibodies will decay after they get vaccine," Grohskopf said. "We do know from studies of populations of people that the immune response will drop over the course of a year and quite possibly not protect throughout a full second season."
There should be plenty of flu vaccine available this year for anyone who wants it. Thanks to their early start, manufacturers estimate they will produce at least 166 million doses. That's 9 million more doses of flu vaccine than last year.
There's also a new type of vaccination this year. In addition to nasal mist and the standard flu shot, one manufacturer is also offering an intradermal vaccine for adults. The intradermal shot deposits vaccine in the skin. Its needle is 90 percent shorter than a standard flu needle.