CDC’s latest weekly influenza data, released Friday, shows that since Oct. 1, there have been more than 13,400 confirmed cases in the United States as of last Saturday. This is more than six times the number of such cases at this time last year. Widespread flu activity has been reported in 23 states.

CDC’s flu forecasters say there’s a 30 percent chance the season will peak around the end of December and a 60 percent chance that the greatest incidence will be by late January, Jernigan said. Generally, flu season peaks near the end of February.

The earlier start this fall, as well as the increasing activity in many parts of the country, “could indicate that this is going to be a more severe season than usual,” said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

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Health officials are urging people to get protected, especially if they’re going to be around young children and the elderly, groups who are most at risk. “If you’re getting ready to see grandmother, call before you go — and make sure you’ve gotten your flu vaccine,” Jernigan said.

The CDC recommends an injectable flu vaccine for everyone 6 months or older as soon as possible. The body takes about two weeks to produce a full immune response.

The other sign that this flu season could be a more serious one: The predominant strain is also the nastiest, H3N2, which causes the worst outbreaks of the two influenza A viruses and two types of influenza B viruses that circulate among people. Seasons where H3N2 dominates typically result in the most complications, especially for the very young, the elderly and people with certain chronic health conditions, experts say.

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“Of the viruses we hate, we hate H3N2 more than the other ones,” Jernigan said.

This strain, which has been around for 50 years, is able to change more quickly to get around the human body’s immune system than the other viruses targeted in this year’s seasonal flu vaccine. H3N2 was the predominant strain three years ago, when the flu season was particularly nasty. “Most of the indicators are showing a pretty substantial rise that may be signaling an early season along the lines of 2014-2015,” Jernigan said.

Even in a good year, the flu vaccine isn’t as good as most other vaccines. Health officials must choose the influenza strains that vaccine makers should target for an upcoming season months in advance, when it is hard to know what strains might be circulating.

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When flu vaccines are well matched to circulating viruses, effectiveness is, at best, around 60 percent. (Measles vaccine, by comparison, is about 97 percent effective with two doses.) In a year when the circulating flu strains closely match the vaccine, that effectiveness rate means that about 3 out of 5 people who get shots are far less likely to become so sick that they require a visit to a doctor.

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For older adults, vaccination not only reduces the chances they will get infected but helps keep them out of the hospital by reducing the severity of infection and related complications, experts say.

A study in Pediatrics this year was the first of its kind to show that vaccination lowered the risk of flu-associated death by half among children with underlying high-risk medical conditions and by nearly two-thirds among healthy children.

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Data collected by the CDC this fall show the majority of viruses collected in the United States to be similar to the viruses used to grow the current vaccine.

Flu vaccines are killed or highly weakened viruses that trigger the body’s immune system to produce antibodies to protect against the real virus. Most U.S. flu vaccines are produced using egg-based technology.

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Scientists have long known that viruses also change in these egg hosts, but recent research suggests that the way flu shots are made may be making them less effective, especially for H3N2.

“When that [H3N2] virus was put into eggs, we see some very tiny changes that make the egg-grown virus look less similar to the viruses that are circulating now,” which could affect the vaccine’s effectiveness, Jernigan said.

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Officials have been especially concerned about H3N2 this year based on surveillance in the Southern Hemisphere — often a clue for what to expect in the United States — because of record-high numbers of laboratory-confirmed cases of influenza there and higher-than-average numbers of hospitalizations and deaths.

Flu virus infections began increasing earlier than usual in Australia and hit historic highs in some areas, according to government data. H3N2 viruses were most common in that country, and interim reports suggest vaccine effectiveness against H3N2 was only 10 percent. The vaccine for the United States has the same composition as the one used in the Southern Hemisphere.

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But Australia vaccinates only the elderly and infirm, who generally don’t respond as well to vaccines. Its approach makes a difference in vaccine effectiveness rates since children have better responses. “It is impossible to say that [the United States] will have a similar 10 percent efficacy against H3N2 with the current vaccine, similar to Australia,” Fauci said.

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CDC officials said a better predictor of vaccine effectiveness for H3N2 is last season's vaccine's effectiveness in the United States. The current vaccine contains the same H3N2 component as last season, which was about 32 percent effective. Most circulating H3N2 viruses that have been tested in the United States this fall are still similar to that H3N2 vaccine virus.

Since 2010, the flu has led to hundreds of thousands of hospitalizations and from 12,000 to 56,000 deaths annually in the United States, CDC estimates.

Even though flu vaccine is not perfect, and some people who get vaccinated may still get flu, experts say data suggest that vaccination may make illness milder.