The current flu season has been more severe than the last three, with more doctor visits and more deaths from flu and pneumonia, federal health officials are reporting.
The season peaked in February, when flulike illnesses accounted for 5.9 percent of doctor visits. Overall, doctor visits for these illnesses were higher than normal for 13 consecutive weeks.
The death rate related to flu and pneumonia was also higher than usual for 13 consecutive weeks; at the worst point, in March, the illnesses were listed as underlying or contributing causes of death in 9.1 percent of deaths. Any rate over 6.9 percent is considered unusually high.
The deaths included 65 children under 18. The youngest was a month old, and the median age was 4.5 years. In each of the three previous flu seasons, 46 to 74 children died.
The statistics were published online on Thursday by the Centers for Disease Control and Prevention.
The main reason for the increased severity this season is that a more virulent type of virus, called A(H3N2), has predominated, said Dr. Dan Jernigan, deputy director of the influenza division at the disease centers. There are many A(H3N2) variants, and this particular one was first identified in Brisbane, Australia, in 2007.
The virus caught vaccine makers off guard. Though the current vaccine does contain an A(H3N2) strain related to the Brisbane variant, it is not exactly the same, making the vaccine less effective than it might have been.
A study found that the vaccine reduced people’s risk of contracting any type of influenza A by 58 percent. But when vaccine strains are well matched to viruses, they can cut the risk by 70 percent to 90 percent in healthy adults. Generally, the vaccines do not work as well in the elderly or in small children.
Normally, if 100 people who are not vaccinated are exposed to the flu, 10 will get sick. But if 100 who are vaccinated are exposed, and the vaccine is 70 percent effective, then only three will get sick.
Flu vaccines generally contain three types of virus, and this year’s vaccine has a second mismatch as well: it is completely inactive against the kind of influenza B virus that has been circulating this year.
Jernigan said mismatches could occur because vaccine makers must decide which strains to include by February, many months before the flu season begins, so that they can make the vaccine in time for autumn. Sometimes, he said, “a virus will emerge that was not even available at the time the decision had to be made.”
The vaccine for the next flu season has been formulated; it will contain three completely different strains from the ones used this year.
Articles published Thursday in two scientific journals, Science and Nature, discussed the evolution and spread of influenza A viruses. Researchers have long believed that new flu viruses originated each year in Asia, and the Science article provided evidence that the theory was correct. Its authors analyzed 13,000 flu virus samples from around the world and traced them to East and Southeast Asia.