Public health officials warn Americans that the flu vaccine may not be as effective against strains of influenza circulating this year. According to the Associated Press, the director of the Centers for Disease Control and Prevention (CDC) Dr. Tom Frieden said that it is possible this flu season may be worse than most at a conference on Thursday.

According to ABC News, changes in the most prevalent strain in the U.S., H3N2, were detected after the vaccinations for this year had already gone into production. Officials could not produce a new batch of vaccination for the mutation because of the quick rate of influenza virus mutations and the process by which vaccinations are produced.

Flu vaccinations take at least four months to produce. Vanderbilt University School of Medicine infectious disease specialist Dr. William Shaffner explained that experts choose three or four influenza strains to protect against via vaccine by February so that production can begin as early as March of each year, according to ABC News. He said that producing an influenza vaccine cannot be “done overnight.”

“Unfortunately about half of the viruses in cases that we’ve identified this season are different than the ones that are included in this year’s vaccine,” Frieden said according to WebMD. Frieden stated that the vaccinations will not be effective for those viruses not included in the vaccine. Frieden added that what experts have seen this year is “concerning” and the flu virus can be unpredictable. However, CDC officials are still encouraging people to get vaccinated because it should provide some protection to other strains or lessen symptoms during infection.

The effectiveness of flu vaccines varies year to year, and flu shots are not considered to be shots that can protect against every flu strain. Last year, the effectiveness—50 to 55 percent— was considered “relatively good” by experts. The LA Times reported that this year’s vaccines are only a match for 48 percent of H3N2 strains.

The CDC said that doctors should be watching for those who are at a higher risk for flu complications including people with asthma, chronic conditions, weakened immune systems, children less than two years old and adults who are 65 or older. According to the CDC, influenza is one of the leading causes of death in America. Approximately 24,000 Americans die on average each flu season. This winter, close to 150 million flu vaccine doses have been distributed.

The news of a new strain was not the only problem with the vaccine for health officials this season. A problem with the flu vaccine in the form of nasal spray was also recently determined by CDC officials, according to the Associated Press.

Experts dealing with vaccines were informed of various study results that found AstraZeneca’s FluMist nasal spray offered little to no protection for children against last year’s most prevalent swine flu strain that was making people ill in a meeting at the CDC in October. Experts are worried that the nasal spray will not be effective this year either since the same version of FluMist is being used. CDC officials predict H3N2 will be the most frequent flu strain that appears this season, though.