Flu strains migrate across the world like flocks of birds. But they garner far more attention. On their typical passage from east to west, flu viruses are monitored by more than 140 national influenza centers in 111 countries as they spread across the continents.

“It all starts in Southeast Asia,” says Craig M. Wilson, MD, professor of epidemiology and director of the UAB Sparkman Center for Global Health. The new strains arise in that area because of the unique congregation of birds, pigs and people.

“That’s the magical combination,” Wilson says. The birds are natural influenza carriers, because the disease does not impact them. “The pigs are the incubators. Then they’re taken to the big Asian markets where there’s a lot of birds, a lot of pigs, and a lot of people,” Wilson says. “We don’t have the same dynamics here, though we do have big bird farms and big pig farms, but not together, and not around a whole lot of people.”

Wilson says there’s no evidence that humans acquire the virus from pigs. “It’s more from the birds. We think the mixing takes place in the pigs,” he says. Then it transfers back to the prime carriers of birds where it’s mutated enough for humans to contract it. Its potency and spread depends on its human-to-human transmission efficiency. Fortunately, many never achieve that sophistication.

When an influenza strain begins to hospitalize humans in their area, the national centers take note. They send those viruses for additional analyses to the five World Health Organization (WHO) Collaborating Centers for Reference and Research on Influenza located in Atlanta, (CDC); London, England; Melbourne, Australia; Tokyo, Japan; and Beijing, China.

Twice a year, WHO consults with the directors of those Collaborating Centers and representatives of key national laboratories. Then, based on their review of the surveillance results, laboratory and clinical studies, and the availability of vaccine virus strains, they recommend what strains to include in the influenza vaccine for that year.

Each hemisphere has their own vaccine. The meeting in February determines the composition of the vaccine for the Northern Hemisphere’s upcoming fall flu season. The September meeting does the same for the Southern Hemisphere’s vaccine.

Then each individual country decides which strains should be included in the flu vaccines licensed in their country. In the United States, the Food and Drug Administration (FDA) makes that determination.

“We have a bit of an advantage here in this country, because the viruses generally arise and become a problem in Asia,” Wilson says. “There aren’t emerging strains in the U.S. But some do get their names here because they’re identified here.” Usually the names reference locations.

This year’s vaccine includes the California/7/2009 (H1N1)pdm09-like virus, an A/Texas/50/2012 (H3N2)-like virus, and a B/Massachusetts/2/2012-like virus. Some of the vaccines also protect against an additional B virus (B/Brisbane/60/2008-like virus).

Another important practical factor in determining the contents of the flu vaccine rests on whether a vaccine strain exists that could protect against the invading virus.

Vaccine viruses must be similar to the invading virus, and, according to the CDC, they must be grown from a clinical specimen in eggs or special pathogen-free chicken kidney cells, but not in any other cell lines.

The vaccine strains must also be tested and available in time for production. That generally means about six months. As a result, occasionally, a threatening virus cannot be identified in time to include in the upcoming year’s vaccine.

That’s what happened this year. A strain, called Switzerland, flared up in September. “So we don’t know how this season will go. There’s a possibility that the vaccine won’t be as effective as previous years,” Wilson says.

But Wilson warns that even without an exact match of viruses, the vaccine still has potency and public health value. “There’s cross reactivity in vaccine viruses, so people are still less likely to get sick if they have the vaccine even if they did not get it exactly right,” he says.

For example, during the 2003-04 influenza season, the vaccine strains were not optimally matched to the strains hitting the country. But in a study of 50-64 year-olds that year, inactivated influenza vaccine effectiveness against laboratory-confirmed influenza was 60 percent among persons without high-risk conditions, and 48 percent among those with high-risk conditions. Even better, the vaccine effectiveness was 90 percent against laboratory-confirmed influenza hospitalization.

Because of that even partial, but significant, protection, Wilson says the more people who are vaccinated, the more the entire population is protected, even if the strains don’t match perfectly between virus and vaccine. “It’s called herd immunity. When enough people have even some immune response, then a virus cannot be as impactful on the population as when no one is vaccinated,” he says.

For the last few years, an estimated 25,000 to 35,000 people have died from flu-related causes in the U.S. “And with a billion people traveling every year, there’s plenty of easy movement of viruses these days,” Wilson says. With that in mind, he adds, people should understand the value of getting vaccinated this year for themselves and for those around them.