Specifically, the flu vaccine has been 50% effective against influenza B/Victoria viruses and 37% effective against influenza A(H1N1)pdm09.
According to former AAFP Vaccine Science Fellow John Epling, M.D., M.S.Ed., of Roanoke, Va., that means the vaccine is about as effective as it typically is in a season when it offers a decent match to circulating influenza antigens.
"The meaning of the effectiveness number gets misinterpreted frequently," he told AAFP News. "While we would all want an even more effective vaccine, it remains the best way we have to prevent flu and its complications."
Because 2019-2020 has been an early and relatively severe flu season and because flu activity continues, Epling said family physicians definitely should continue to offer vaccinations to all unvaccinated patients.
"Remember that influenza vaccination not only reduces the incidence of flu, but also reduces the severity and hospitalization associated with flu," he noted. This is particularly important in children and adolescents, given that they have been disproportionately affected by severe illness this season
The CDC's MMWR emphasized that this year's vaccine has thus far substantially protected children and adolescents ages 6 months to 17 years, with 55% vaccine effectiveness seen in this population.
"We have a vaccine this year that seems to work better in this age group, so it is very important for family physicians to inform families that there is an effective way to reduce the chance of severe illness in their children," said Epling.
In addition to these benefits, a reduction in influenza-associated illness can lead to less time away from school for kids and away from work for adults (those who get sick themselves or must take care of sick kids).
Based on data for 4,112 children and adults enrolled in the U.S. Influenza Vaccine Effectiveness Network who reported acute respiratory illness from Oct. 23 to Jan. 25, the CDC determined that the 2019-20 influenza season began early with predominantly influenza B/Victoria virus circulation. This was followed by increasing A(H1N1)pdm09 virus activity, and detection of both viruses is ongoing.
The report noted that severe illness markers, including laboratory-confirmed influenza-associated hospitalization rates among children and adolescents and among adults ages 18-49, are higher than at this time in recent flu seasons, including the 2017-2018 season, during which influenza A(H3N2) viruses predominated.
So far this season, 92 influenza-associated deaths have been reported in children and adolescents younger than 18 -- the highest number of deaths at this point in the season since reporting began for the 2004-2005 season, the CDC said.
Among the patients with acute respiratory illness enrolled during the analysis period, 26% tested positive for influenza virus infection using real-time reverse transcription-polymerase chain reaction, including 17% for influenza B viruses and 9% for influenza A viruses, with five patients testing positive for both, according to the report.
And as of Jan. 25, the CDC said it has genetically characterized 177 influenza B/Victoria viruses from U.S. Flu VE Network participants; 97% belonged to genetic subclade V1A.3 (different from the V1A.1 subclade that includes the 2019-2020 B/Victoria vaccine reference strain [B/Colorado/06/2017]) and 3% belonged to V1A.1.
All 32 genetically characterized A(H1N1)pdm09 viruses were from genetic group 6B.1A, which includes the 2019-2020 A(H1N1)pdm09 vaccine reference strain (A/Brisbane/02/2018).
CDC researchers stressed in the MMWR that more effective influenza vaccines are needed, and they said that government researchers are working to achieve that goal.
"Influenza is a challenging annual public health threat because the virus changes in unpredictable ways, new viruses emerge, and it takes time to produce the massive quantities of vaccine needed to protect the public," Epling said.
An executive order President Donald Trump signed in September prioritizes efforts to increase uptake of influenza vaccination, improve production efficiency of the vaccine and research new vaccines that can provide longer-lasting coverage, he added.
"It created a National Influenza Vaccine Task Force to work across multiple agencies to meet these goals," Epling noted. "We await the report from this task force, but I believe the additional focus and prioritization are potentially very helpful."
The CDC's report also noted that antiviral medications are important as an adjunct to flu vaccination.
Antiviral medications can be a useful additional tool to fight influenza, Epling said, but they're limited by a lack of hard evidence that they improve important outcomes such as hospitalizations and deaths.
"They can improve symptom duration by a little less than a day, so I discuss the limitations with my patients who see me for flu, and tend to prioritize it for those who are at increased risk of bad outcomes with flu -- those with asthma, heart disease, etc. -- hoping that it will help," he said.
As for limitations of the CDC's interim estimates of flu vaccine effectiveness, the MMWR said the sample size was insufficient to estimate overall VE against illness associated with A(H3N2) virus infections.
"This is an interim report, so things may change as we get more experience with all the different strains of the virus," Epling said. "A(H3N2) could have a peak later in the season, or it may not, so we will have to wait for the final estimates of vaccine effectiveness."
Finally, Epling said this interim report doesn't change what family physicians already are doing in practice.
"We should continue to offer influenza vaccine to all our patients age 6 months and older," he said. "If anything, the information about the more severe illness in children and adolescents and the evidence of good vaccine effectiveness in this population should bolster our efforts to get our patients vaccinated."