Flu has killed 20 children nationwide so far this season

The flu was widespread in 47 states as of the first week of January, and this flu season has so far killed a total of 20 children, the Centers for Disease Control and Prevention said Friday.

Currently, 24 states and New York City are showing high levels of flu activity. Sixteen states are reporting moderate levels. The West Coast is the one area of the country that hasn't seen heavy flu activity yet, said Tom Frieden, director of the CDC.

Though that's already starting to shift in San Francisco, said Henry Chambers, chief of infectious disease at San Francisco General Hospital. The number of cases has been slowly creeping up over the past week and now they're seeing a surge. The hospital "is at lift off" when it comes to flu cases, he said.

About 4.3% of visits to a doctor or a clinic were for influenza-like illnesses last week, above the national baseline of 2.2%, CDC said in its weekly FluView report.

Flu activity may have peaked in some areas, with slight declines in the South and Southeast. However, it's also possible that's just a quirk of reporting because of the winter holiday season, Frieden said.

"We'll see in a week or two whether we have actually crossed the peak," he said.

This year's flu vaccine appears to be about 62% effective against the currently circulating strains, according to CDC. This is "moderate effectiveness," according to CDC's report, and slightly above the usual level of effectiveness for flu vaccine based on previous research.

Although that may seem low, experts point out that not getting a flu shot means you have 0% protection.

"Vaccination is the single-most important step you can take to protect yourself," Frieden said. "If you've gotten the flu vaccine you're 62% less likely to need to go to the doctor to get treated for the flu."

It's also not too late to get vaccinated. "Better late than never," said Frieden, noting that flu season typically lasts for 12 weeks.

Vaccination is crucial for not only personal but also for public health. It offers substantial protection for the individual. In addition, people who have been vaccinated and later go on to get the flu appear to have much milder cases. And those same people also shed less flu virus, making it less likely they'll pass it along to those around them, said Arnold Monto, a professor of epidemiology at the University of Michigan in Ann Arbor, Mich., and a member of the Infectious Diseases Society of America's Influenza Advisory Group, which works with CDC.

About 37% of Americans had gotten vaccinated against the flu as of mid-November, said Joseph Bresee, chief of CDC's epidemiology and prevention branch. CDC hopes that number will reach close to 50% by the end of the season, but doesn't yet have more recent figures. Compared with past years, we're about on track with vaccinations; usually about half of Americans end up getting a flu shot by the end of the season, he said.

Anecdotal reports from some medical centers show some people who had documented flu shots later coming down with the flu. Robert Glatter, an emergency medicine physician at Lenox Hill Hospital in New York, is seeing indications that this year's vaccine may not be as protective as had been hoped, especially against Influenza B, one of three flu strains currently circulating. "This may be contributing to some patients' profound and severe symptoms associated with the 2013 outbreak," he said.

In addition to the flu, there's also a fair amount of norovirus circulating in the country, said CDC's Bresee. Norovius is a gastrointestinal illness that causes diarrhea, vomiting and nausea. While flu can cause vomiting, stomach cramps and mild diarrhea in some people, those symptoms in norovirus tend to be more intense and "explosive," in the words of medical experts.

Michael Osterholm, director of the Center for Infectious Disease Research and Policy, says a careful analysis of the data shows that the flu vaccine's "match" has little to do with its effectiveness. Even flu vaccines that are well-matched for the flu strains in circulation may still offer poor protection, says Osterholm, also a professor at the University of Minnesota School of Public Health.

A report released four months ago by the the Center for Infectious Disease Research and Policy at the University of Minnesota found that while many studies put the efficacy of flu vaccine for healthy adults at between 70% to 90%, when they looked only at well-designed and rigorous studies it was closer to 59% for healthy adults between 18 and 64 years of age. The intranasal vaccine, sold as FluMist, is about 83% effective for children ages 6 months to 7 years, the report says.

The flu vaccine used in the United States is based on the mix of flu viruses circulating in the Southern Hemisphere the season before. Every year, virologists at the World Health Organization look at the mix and attempt to predict what will appear in the Western Hemisphere later in the year.

They announce their decision in April, just as the previous flu season in the Northern Hemisphere is wrapping up. Vaccine manufacturers have about five months to produce vaccine for the next season, and vaccination begins in August and September, said Michael Jhung, an officer in the Centers for Disease Control and Prevention's influenza division.

The best guess for this year's season was a mix of H3N2, H1N1 and Influenza B. The H3N2 and H1N1 in the vaccine "look a lot like" the flu currently in circulation and the Influenza B strain is about a 70% match, Jhung said. "That's not bad," he said.

However, "we have an influenza strain in circulation, which is causing 6% to 10% of infections, that's not included in the vaccine," said William Schaffner, an expert on infectious diseases at Vanderbilt University School of Medicine in Nashville.

While the flu shot is much more protective than nothing, it still doesn't have much power to produce true "herd immunity," Osterholm said. Herd immunity is achieved when so many people are immunized with an effective vaccine that the community's viral load drops, protecting even the unvaccinated.

Osterholm encourages people to be vaccinated, but says it's also important for people to realize how mediocre the current vaccines are. "The No. 1 deterrent to getting new flu vaccines is the perception that the current ones are good enough," he said.

Osterholm said the world needs "next generation" vaccines to provide broader, longer-lasting protection from the flu. But very few of these "game-changing" vaccines are being developed, partly due to the expense and financial risk.

Of the 177 influenza vaccines in clinical trials around the world, all but 13 are made in the traditional way, Osterholm said. None of them is supported by the U.S. government.