The flu is a substantial issue in America right now. If you have not yet been affected by it, odds are, someone you know has. According to the Centers for Disease Control and Prevention, this is the first time in 13 years that the flu has made a widespread appearance in the entire continental United States.

But what makes this year’s outbreak of the flu so different from previous years, and why is the vaccine less effective this season?

In order to answer these questions, we must first study how both influenza and its vaccine work.

Each season, different strains of influenza circulate. This year, the most prevalent strain is H3N2, a strain of the influenza A virus that seems to cause more health problems and is much more difficult to prevent than other strains.

Scientists still don’t know the exact reason, but they have found that strains involving the H3 virus typically lead to more hospitalizations and deaths than other viruses, such as influenza B.

According to the CDC, 90 percent of flu cases this year involve the H3N2 strain, meaning that the current flu season could get quite a bit nastier than usual.

We don’t exactly know what the rest of this flu season will look like, but if it is anything like what happened in Australia this season, it could be ugly.

Just like in America, the H3N2 strain was prevalent in Australia, and it contributed to over two-and-half times the amount of flu cases from the previous year.

In addition to being particularly threatening, the H3N2 strain of influenza is extremely difficult to prevent with vaccines. Flu vaccines are designed to focus on three or four strains of influenza and protect people against those particular strains.

Every year, public health agencies conduct a great deal of research and decide which strains and mutations they think will circulate in the coming flu season.

Obviously, this strategy is not perfect by any means, but researchers have found that flu seasons containing the H3N2 strain are the seasons in which the vaccine is the most imperfect and the least effective.

One of the major reasons that H3N2 is so difficult to find a vaccine for is that it mutates at a much faster rate than other viruses.

Scientists also typically run into issues while trying to grow H3N2 in eggs, where the viruses for flu vaccines are produced, due to the fact that changes to the virus can decrease the effectiveness of the vaccine.

It is not too late to receive a flu shot from your local pharmacy or clinic. This year’s flu season will certainly last a few more weeks, and maybe even a few more months.

Although receiving the flu vaccine does not completely guarantee your health, vaccines carry very little risk, and the potential benefit of avoiding severe illness is well worth the risk.