Drug companies in the US have amped up this year’s flu vaccine in the hope it will be more effective than last year’s defense against the winter ailment. This is the first year that all of the vaccines produced by GlaxoSmithKline (GSK) – one of the six companies that make doses of vaccines in the US – include an extra strain of the flu. According to public health experts, the outlook for this year’s batch of vaccines is that it is likely to reduce the number of flu cases because it is better matched to the strains of influenza present this season.

Here’s everything you need to know about this year’s flu vaccine.

What’s different about this year’s flu vaccine?

“The vaccine strains are matching what is circulating,” said Dr Brian Koll, professor of infectious diseases at Mount Sinai Hospital in New York.

All of GSK’s flu shots for the US will be four-strain vaccines this year, consisting of two strains of influenza A and two of influenza B. Influenza A tends to be more virulent and shows up more in the fall and winter, Koll said, while influenza B tends to appear more near springtime and to affect fewer people. Previously, most vaccinations contained only three strains.

Leonard Friedland, a vice president of scientific affairs and public health at GSK said: “Our best knowledge suggests that the strains selected for the vaccine are the right strains for this year.”

Friedland added that a four-strain vaccination will provide “the broadest possible coverage.”

Not all of these strains are new: two of the four strains in this year’s vaccine were also in last year’s. The decision of which strains to change and which strains to keep in the vaccine “depends on what’s circulating”, Koll said.

How effective will the vaccine actually be?

Although temperatures have started dropping, the flu season has gotten off to a slow start. According to the US Centers for Disease Control and Prevention’s influenza reports, activity has been low and limited so far this year. The flu is now believed to be widespread in Guam, with regional activity in Puerto Rico, localized activity in seven states and sporadic instances in 38 others.

The CDC described last year’s flu season as “moderately severe”, and noted it was exacerbated by a vaccine that was only about 19% effective. But according to experts, that was an anomaly, not a trend. “They get it right about 80-90% of the time,” said Art Reingold, head of epidemiology at the University of California, Berkeley, School of Public Health.

“No one can tell you with certainty whether we’ll have a good match for what’s circulating and causing seasonal influenza,” Reingold said. “And if that sounds unsatisfying, that’s still the best we can do.”

Mark Schleiss, director of the Division of Infectious Diseases and Immunology at the University of Minnesota Masonic Children’s Hospital, advocated strongly for the annual use of the current flu vaccines, but admitted it is not a perfect system. “The flu vaccine that we give is not optimal. We need a better vaccine. We need a vaccine that can protect for more than one year. We need a vaccine that can protect against more than one strain,” he said.

While it seems unlikely at this point, the predominant flu strains could shift course, rendering this year’s vaccine less effective than expected. Even if that happens, it’s possible that the vaccine could at the very least decrease the severity of its symptoms and reduce the number of hospitalizations that inevitably occur for flu-related pneumonia. Schleiss said this is because the wrong strain of influenza vaccine can still trigger an immune response to other strains.

When can we expect a universal flu vaccine?

Researchers are still working to determine whether it’s possible to develop a universal vaccine that will protect from all flu strains. The idea is to provide the vaccine to infants for lifelong protection, but no one yet knows whether it will work. Thus far, it’s not possible, because flu viruses are constantly mutating and evolving. Schleiss said the key to a universal flu vaccine is to find a strain of influenza that provides a strain-independent immune response that can protect against all flus regardless of mutations and other changes.

Who makes the flu vaccines?

The influenza vaccine is the product of a partnership between international health groups, national agencies and corporations. Every day, the World Health Organization (WHO) and other agencies collect samples from patients with respiratory symptoms at more than 100 reference laboratories across the globe. In the US, 240 laboratories also provide the results of flu tests to the CDC.

The WHO consults with the CDC – as well as its counterparts in the United Kingdom, Australia, Japan and China – to determine the strains that are most likely to spread during flu season. Representatives of these groups, along with other agencies that track the flu, meet at the WHO in Geneva each February. This year, the WHO announced the flu vaccine composition for the Northern Hemisphere on 26 February, and the US Food and Drug Administration’s (FDA) Materials, Vaccines and Related Biological Products Advisory Committee endorsed it in March.

In the US, the CDC and the FDA provide the seed viruses for the vaccine to six drug companies, including GlaxoSmithKline, that are authorized to manufacture vaccines. These companies, in turn, grow the seed viruses in fertilized chicken eggs, deactivate the viruses and produce vaccines, Friedland said.

How many flu vaccines will be made?

According to Friedland, GSK expects to manufacture 37m doses of vaccine this year. Overall, the CDC estimates that drug companies will produce 171m-179m doses. Nearly 141m doses have already been distributed by the manufacturers.

When should I get a vaccine?

Koll said flu season is likely to ramp up in December, but adds that New York – where he works – has experienced a warm fall, so the illness’ full-force arrival may be slowed. Still, it’s not too early to prepare. Full coverage for patients who receive the vaccine kicks in two weeks after the dose is administered, so public health advisors suggest getting the shot as soon as possible. The CDC recommends the flu vaccine for everyone older than six months.