The current seasonal flu vaccine protects about 45% of recipients against the circulating strains, according to midseason estimates of vaccine effectiveness (VE) published today in the Centers for Disease Control and Prevention's (CDC's) Morbidity and Mortality Weekly Report.
A higher percentage, 58%, was seen in the first estimates on effectiveness during Canada's flu season, which were published in a separate study today.
US protection 50% against flu B
The CDC said the US flu season started early—the earliest seasons since the 2009 H1N1 flu pandemic year—in the first week of November, and is still currently above baseline activities.
To conduct the study, CDC researchers and their state partners used data collected from five study sites (Michigan, Pennsylvania, Texas, Washington, and Wisconsin), on patients aged 6 months or older seeking outpatient medical care for an acute respiratory illness (ARI) with cough within 7 days of illness onset. Any patient who took antiviral medications was not included in the study.
Among the 4,112 ARI patients enrolled in the study from Oct 23, 2019, to Jan 25, 2020, a total of 1,060 (26%) tested positive for influenza virus infection, including 691 (17% of the total patients) for influenza B viruses and 374 (9%) for influenza A viruses. A total of 37% of flu-positive patients had received the 2019-20 seasonal influenza vaccine, compared with 55% who were unvaccinated.
Overall, the adjusted VE was 45% against influenza A and B viruses combined, 50% against influenza B/Victoria, and 37% against A/H1N1, the authors said. Flu VE for adults ages 18 to 49 against H1N1 was especially low (5%; 95% confidence interval, − 45% to 37%), and younger adults and children seemed to get more protection from the vaccine against all circulating flu strains.
"Interim VE estimates against influenza A(H1N1)pdm09 viruses among children and older adults are consistent with average VE for influenza A(H1N1)pdm09 viruses reported previously," the authors wrote.
"Among adults aged 18–49 years, the interim VE estimate against influenza A(H1N1)pdm09 was low and not statistically significant. Additional enrollment during the season while A(H1N1)pdm09 viruses circulate will determine whether VE against A(H1N1)pdm09 in this age group is lower than during previous seasons and will help evaluate potential contributing factors to lower than expected effectiveness."
The beginning of the US flu season was dominated by influenza B/Victoria, but since December, cases of H1N1 flu have increased.
Despite the protection, the authors said markers of severe illness, including laboratory-confirmed flu-associated hospitalization rates among children and adolescents, are higher than in recent seasons, including the severe 2017-18 season.
The authors also noted that the 92 flu-associated deaths in children are the most at this point in the season since reporting began in 2004-05, other than the 2009 pandemic season, and stress the importance of still getting vaccinated if people have not yet done so.
Solid protection in Canada
The Canadian flu season was also defined by early influenza B/Victoria activity, but the flu vaccine has proved more effective so far, according to a Eurosurveillance study. Patients 1 year old and older presenting at study sites in Alberta, British Columbia, Ontario and Quebec were included in the study.
Of the 2,808 specimens tested, 1,411 (50%) were influenza test-positive, including 731 (52%) influenza A and 683 (48%) influenza B viruses, with three influenza A and B co-infections. Of the influenza A cases, (77%) were H1N1 and 23% were H3N2. Among the influenza B detections, 99% were B/Victoria.
Overall, VE against any strain was 58%, including 74% in children ages 1 to 19 years, and 18% in adults 65 and older. VE was 44% against H1N1, 62% against H3N2, and 69% against influenza B. As in the United States, the vaccine provided better protection in children and adults under 50 seemed to get more protection from the vaccine.
"The mid-season VE of 44% we report for A(H1N1)pdm09 viruses during the 2019/20 season is lower than we reported mid-season in 2018/19 (72%) , 2015/16 (64%)  or 2013/14 (74%)," the authors wrote.
They conclude, "Interim results from Canada's SPSN indicate that the 2019/20 influenza vaccine has provided substantial protection against medically-attended influenza illness during a season characterised by an approximately equal mix of influenza A and B viruses, a substantial proportion of which were genetically and antigenically mismatched to vaccine."
They also suggest the substantial VE despite a mismatch between the circulating strains and the vaccine strains, "invites exploration of other factors potentially contributing to VE."