News published online Thursday by the Centers for Disease Control and Prevention (CDC) includes statistics for this year’s influenza season. It was the worst of the last three seasons.

Flu season starts in the fall and peaks in February. During this year’s February peak, flu-related symptoms were the reason behind 5.9% of all doctor visits in the US, a rate higher than in the previous 13 consecutive weeks.

The death toll seems to have peaked in March, however, with 9.1% of all causes of death associated with flu or pneumonia. Healthcare officials consider 6.9% to be an unusually high rate.

Sixty-five children aged 1 month to 18 years have died so far in this year’ flu season. The median age at death among childrern has been 4.5 years. In the three previous years, between 46 and 75 children died each year.

Dr. Dan Jernigan, deputy director of the CDC’s influenza division, identifies a more virulent form of virus that has predominated this year’s flu season. This one, A(H3N2), was first identified in 2007 in Brisbane, Australia. There are numerous variants of this virus but this particular strain was not identified in time to be included in this year’s batch of influenza vaccines although a variant of it was included.

Jernigan cites another reason for such a fierce flu season this year. We’re seeing the presence of an influenza B virus but it was not included in the vaccine formulations used last fall. The vaccines are completely ineffective against this form of virus.

Generally, flu vaccines target three strains of virus and each year’s formulation is different from the one administered during the previous year. In order to have an ample supply of vaccines ready when flu season begins in the fall, vaccine makers must begin production as early as February. The virus strains that will be in this coming fall’s vaccine have already been selected and each one is different from the three used this year.

With the two mismatches in this year’s flu vaccines, the risk of getting the flu, even if vaccinated, was reduced by about 58%. Closer matches offer 70% to 90% reduced risk of illness. In any case, small children and the elderly typically receive less protection from flu vaccinations than the rest of the general population.

In any given year, 10 out of every 100 unvaccinated people are likely to get the flu. When a vaccine is 70% effective, only 3 out of 100 vaccinated people is likely to contract the flu.

In related news, both Science and Nature, two scientific journals published flu-related articles on Thursday. The article in Science confirms a theory of virus origin and traces its spread around the world.

A long-held belief in the scientific community is that new flu viruses originate each year in Asia. The authors of the study analyzed 13,000 influenza virus samples gleaned from around the world. They then traced their points of origin to find that they all came from East and Southeast Asia.

Once active in this area, the virus then spreads to Europe and North America, where it is frequently detected six to nine months later. After another few months, the virus reaches South America, where it eventually dies out.

The CDC estimates that, during any given year, 5% to 20% of the US population will get the flu. More than 200,000 people with the flu will require hospitalization and nearly 36,000 will die from complications of influenza.