In a flu season that’s been deemed the worst in a decade, the Centers for Disease Control and Prevention (CDC) say the flu vaccine is only 25 percent effective in the US.

Low rates of vaccine effectiveness have already been reported in Canada – 17 percent – and Australia, just 10 percent.

This year’s vaccine reduces your chances of getting the flu by a third, but is just 25 percent effective against the predominant H3N2 strain in the US.

The study looked at more than 4,500 children and adults with acute respiratory illness (ARI) associated with the flu between last November and February at five US hospitals. The report notes real numbers could be higher considering not everyone goes to the doctor to confirm cases of the virus.

They found vaccines are preventing just 36 percent of influenza infections that send people to the doctor, and just 25 percent of those caused by influenza A (H3N2).

It’s one of four viruses associated with the flu – others include Influenza A (H1N1) and B strains (Victoria or Yamagata lineages) – and it’s posing the greatest challenge as it makes up 75 percent of verified flu cases in the US.

The effectiveness varies from season to season, but in past years it has generally been higher against both the H1N1 and H3N2 viruses.

The CDC has been tracking the vaccine's effectiveness since 2004, and while it varies from season to season, in past years it has typically reduced the risk of flu by between 40 and 60 percent.

The CDC still recommends the flu vaccine as a way to reduce the risk of being infected by the virus and experiencing serious complications as a result.

“Vaccination will still prevent influenza illness, including thousands of hospitalizations and deaths,” said the CDC.

The vaccine works by stimulating the body’s immune system to make antibodies that attack the virus, and it can take up to 14 days for immunity to fully build. If exposed to the flu after receiving the vaccine, the body’s immune system will recognize the virus and immediately produce the needed antibodies to fight it. 

Each February the World Health Organization meets to address the strains of flu virus that will most likely circulate in the Northern Hemisphere the following winter. Starting in March, vaccine manufacturers begin producing flu vaccines based on those recommendations.

Viruses evolve constantly and officials say the replacement of viruses in the vaccine is necessary to keep it effective.

The report says vaccine rates are up in children aged six months to eight years, and are 59 percent effective.  

The flu vaccine also reduces the severity of symptoms for those who contract it, and with a season that’s expected to last several more weeks, officials say it’s not too late to get vaccinated.