(CNN) - Flu season is still raging in the United States, yet already it ranks among the most difficult in recent years. One reason: This year's vaccine has been just 36 percent effective against both A and B virus strains, the U.S. Centers for Disease Control and Prevention estimated in Thursday's Morbidity and Mortality Weekly Report.
"You're 36 percent less likely to get the flu and see the doctor if you get a flu shot," Health and Human Services Secretary Alex Azar said Thursday.
"If a young child gets a flu shot, he or she is 59 percent less likely to get the virus and have to go to the doctor," Azar said. "Getting the flu shot is the same kind of sensible precaution as buckling your seat belt. If you got the flu shot but you end up catching the flu, it could be less severe and less likely to land you in the hospital."
Surgeon General Dr. Jerome Adams also recommended the flu shot and urged parents, in particular, to get their children vaccinated.
"The flu vaccination is safe," he said. "It is still your best defense."
He noted that three-quarters of the 63 children who have died of flu this season had not been vaccinated.
Different strains, different effectiveness
More than two-thirds of flu sickness this season -- 69 percent -- has been caused by the H3N2 strain, an influenza A virus.
H3N2 viruses "are often linked to more severe illness," acting CDC Director Dr. Anne Schuchat said Thursday.
This year's vaccine showed just 25 percent effectiveness against this strain at midseason, according to the CDC.
Though this percentage is low, it compares favorably to a recent midseason study from Canada that estimated 17 percent effectiveness against the H3N2 strain for that country's flu vaccine. Australia's end-of-season estimates found even less effectiveness against the same strain for that nation's vaccine: a mere 10 percent.
And, as Schuchat noted, the vaccine was 51 percent effective against this strain in young children. "The vaccine's performance in children was better than we expected," she said.
Influenza activity in the US began in October and increased in early November, then rose sharply from December through February 3, according to a separate CDC report released Thursday. Although H3N2 has predominated, influenza A(H1N1) and influenza B viruses have also been in circulation across the country.
Against these two less-prevalent flu strains, the vaccine earned higher marks of effectiveness. The CDC's midseason estimates of vaccine effectiveness against H1N1 is 67 percent and against B viruses 42 percent.
Food and Drug Administration Commissioner Dr. Scott Gottlieb explained in a statement Thursday that the flu virus has the ability "to change rapidly as it circulates," which makes it difficult to develop an effective vaccine. He also said most doses of vaccine are made using chicken eggs, though some are made in cells.
"This year, the cell-based influenza vaccine appears to have somewhat better effectiveness in preventing influenza than the egg-based vaccine," Gottlieb said, based on a preliminary CDC analysis. This knowledge should help improve next year's vaccine effectiveness, he said.
"We know in general that people that are younger and healthy can have better responses to vaccines than people who are older or very young," said Dr. Michelle Barron, medical director of infection prevention and control at UCHealth University of Colorado Hospital. She was not involved in the CDC research. "There is a vaccine specific for people who are older that gives you a higher dose." This is believed to work a little bit better in that age group.
Illness, hospitalizations and deaths
Forty-six of the 50 states, the District of Columbia, New York City and Puerto Rico have been experiencing high levels of influenza-like illness during the week ending Feb. 3, the CDC reported. By comparison, only 31 states/jurisdictions experienced high illness in a single week during the two previous seasons known to be severe: the 2012-13 and 2014-15 seasons.
The baseline (or expected level) of flu-related doctor's office visits for an average week at this time of year is just 2.2 percent. The nation as a whole exceeded this rate during the week ending Nov. 25 and has remained at or above it -- ranging as high as 7.7 percent -- for 11 consecutive weeks.
Through Feb. 3, flu-related hospitalization rates are 40 per 100,000 children up to 4; 10 per 100,000 children between 5 and 17; 18 per 100,000 adults between 18 and 49; 63 per 100,000 adults between 50 and 64; and 264 per 100,000 adults 65 and older.
The CDC also found that about 86 percent of hospitalizations this season were associated with influenza A infections, about 13 percent with influenza B infections and less than 1 percent with co-infections. Of those patients hospitalized with an A strain infection, nearly 86 percent were caused by the H3N2 strain.
Pneumonia and flu-related deaths rose sharply in the first weeks of 2018, accounting for 10 percent of all deaths during the week ending Jan. 20. The CDC said it anticipates that these deaths will continue to increase for several weeks and might exceed the peaks of recent severe seasons.
And nearly half -- 46 percent -- of the 63 children who have died as of Feb. 3 due to flu-related causes were otherwise healthy, the CDC said.
The CDC tested the currently approved antiviral medications oseltamivir (brand name Tamiflu), peramivir (Rapivab) and zanamivir (Relenza Diskhaler) against the currently circulating flu strains. Four of 376 H1N1 strains tested against oseltamivir and peramivir were resistant to both drugs. Yet all of the 265 H1N1 strains tested were killed by zanamivir. All three antiviral medications worked against H3N2 strains and various strains of B viruses.
Adams said everyone needs to "remember the three Cs":
Clean your hands frequently. Cover your cough and sneeze. Contain your germs.
"If you feel sick, please stay at home and rest," he said, adding that anyone who becomes sick should contact their health care provider and, if prescribed antiviral medication, begin it as soon as possible.
"Getting vaccinated does not mean you won't get sick, but it can lessen your illness," Adams said.
Azar agreed: "Go get a flu shot. Do it for yourself, your family and your community."
Schuchat said, "We could be looking at several more weeks of intense activity. It's been a scary flu season."
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