Americans got little benefit from last season's flu shot — the vaccine was only about 19 percent effective, according to the Centers for Disease Control and Prevention. That's largely because one of the flu strains that was used to make the vaccine did not match well with the actual flu strains that were circulating.

Now, new research shows that a single mutation in that strain is what caused this poor match.

In the study, researchers used blood from ferrets and sheep that had been infected with a live version of the H3N2 virus. (A killed version of this virus was used in the flu vaccine.) They tested the antibodies the animals produced against an array of flu viruses that contained different mutations,

The researchers identified one mutation that when present, made the animals' antibodies respond much more weakly to the viruses.There was as much as a fourfold decrease in how effective the antibodies were in fighting the virus that had this mutation, the researchers said.

Subsequent experiments using blood from people who had received the seasonal flu vaccine produced similar results, according to the study.

"There was only one mutation that really had a profound effect," said Scott Hensley, the lead researcher on the study and an assistant professor at the Wistar Institute, a medical research center in Philadelphia. The mutation was found in the gene for the virus's hemagglutinin molecule, the "H" part of the H3N2 strain.

Since the researchers already knew the vaccine was a bad match this year, looking back and identifying the problem was the easy part, Hensley said. "What we'd like to do is be able to look ahead and try to predict how the virus might mutate in the future,and be able to make predictions if next year's flu vaccine will be effective," he told Live Science. But that is much harder to do. [The 9 Deadliest Viruses on Earth]

Unlike some viruses, influenza's genetic code regularly mutates as it replicates, so the virus is continuously changing; flu experts refer to this process as antigenic drift. For this reason, the flu vaccine needs to be created anew each year. In February, the World Health Organization recommended that the flu vaccine for the 2015-2016 flu season include an updated strain of H3N2.

The new findings are important because they show researchers where the "hot spots" in the virus are, which point to how the virus mutates to get around the immune system, said Andrew Pekosz, an influenza expert at the Johns Hopkins Bloomberg School of Public Health who was not involved in the study.

Pekosz said that he's optimistic that next year's flu vaccine will offer more protection against the H3N2 strain. But he's concerned about another flu strain, called H1N1, for next year's flu season. Protection against H1N1 is also included in the seasonal flu vaccine.

"The worry right now is the H1N1 component," he adds. "That virus hasn't really changed in a very long period of time. Much longer than we'd expect." That means, he says, that a change might be in store for it.

Copyright 2015 LiveScience, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.