Researchers brought more bad news today on the performance of this winter's flu vaccine—which does not match well with the dominant circulating strain—saying it has shown no significant effectiveness in preventing flu in the United Kingdom or in preventing flu-related hospitalizations in Canada.
The British team estimated the vaccine's effectiveness in preventing medically attended flu at just 3.4%, while the Canadian group put its effectiveness for preventing flu-linked hospitalizations in the negative range: –16.8%. The findings were published in today's Eurosurveillance.
Those mid-season estimates are similar to another Canadian estimate released last week but lower than a US estimate issued in mid-January. The estimate a week ago from the Canadian Sentinel Physician Surveillance Network put the vaccine effectiveness (VE) at –8%. And on Jan 15 the US Centers for Disease Control and Prevention reported a mid-season VE estimate of 23%. Both estimates pertained to flu in outpatients, not hospital patients.
A more typical VE estimate for seasonal flu vaccines is in the 50% to 60% range, at least in healthy, working-age adults, with lower numbers in the elderly. Health officials have attributed the vaccine's poor performance this winter to a mismatch with circulating influenza A/H3N2 viruses, which are overwhelmingly dominant this season. About two-thirds of H3N2 viruses from US patients have differed from the H3N2 strain in the vaccine.
The British study is based on data from five sentinel flu surveillance programs. The researchers used a test-negative case-control design, in which patients seeking care for a flu-like illness (ILI) are tested for flu and their vaccination status is determined. Vaccination rates for infected and uninfected patients are then compared.
The British team identified 1,341 patients who met their criteria, of whom 312 tested positive for flu by polymerase chain reaction and 1,029 tested negative. The vaccination rates were 20.8% (65 of 312) for the cases and 17.2% (177 of 1,029) among the controls.
Of the 312 cases, 271 had H3N2 infections, 14 had H1N1 viruses, 16 had type B viruses, and 11 had type A viruses that were not subtyped.
Given the numbers, the researchers estimated an adjusted overall VE of 3.4% (95% confidence interval [CI], –44.8% to 35.5%). VE against H3N2 viruses was estimated at –2.3% (CI, –56.2% to 33.0%).
The authors observed that analyses of H3N2 viruses collected in the United Kingdom during the study showed that there is "a clear antigenic mismatch between the northern hemisphere H3N2 vaccine strain and the circulating variant in winter of 2014/15." They added, "The full picture of virological variation requires further detailed analysis, not possible at this stage of the 2014/15 season."
The report says the UK findings represent the lowest VE measured there in the past decade with the test-negative case-control method. But the authors note that the wide confidence intervals point to uncertainty and make it impossible to say the vaccine has no effectiveness.
They add that UK flu incidence has been lower this winter in areas where school children have been vaccinated under a pilot program compared with other areas, suggesting a possible effect of the vaccination program.
The findings point up the importance of other flu countermeasures this season, especially the early use of antivirals for postexposure prophylaxis and treatment in vulnerable groups, the researchers conclude.
The Canadian study comes from the Serious Outcomes Surveillance (SOS) Network of the Canadian Immunization Research Network (CIRN), consisting of 15 hospitals in five provinces. The researchers used a test-negative case-control design to assess VE for preventing hospitalizations related to lab-confirmed flu infections.
Between Nov 15, 2014, and Jan 10, the team identified 600 hospitalized flu case-patients and 471 hospitalized test-negative controls. Of 599 cases in which viruses were typed, 593 were influenza A. And of 216 isolates that were subtyped, H3N2 accounted for 214 (99.1%).
The team found that 399 of the 600 case-patients (66.5%) had been vaccinated, as compared with 300 of 471 (63.7%) of the controls. From that, they came up with an overall VE, adjusted for age and the presence of comorbidities, of -16.8% (CI, −48.9% to 8.3%). The estimate was lower in elderly (65 and older) adults, at −25.4% (CI, −65.0% to 4.6%), and higher among younger adults, at 10.8% (CI, −50.2% to 47.0%). The report says that 69% of patients in the study were older than 75.
The researchers write that more than 99% of circulating H3N2 viruses characterized in Canada so far this season were not well matched to the H3N2 component of the vaccine, versus about 67% in the United States. They say this helps explain why estimated VE in Canada is lower than the US CDC estimate of 23% effectiveness against medically attended flu in the community.
The recent US VE estimate and the earlier Canadian VE estimate both pertained to vaccine performance against medically attended flu among patients of all ages in the community, "and thus might be predicted to be higher than our estimates of VE in the prevention of influenza-associated hospitalisation in predominantly elderly patients with medical comorbidities," the authors observe.
UK study in Feb 5 Eurosurveillance
Canadian study in Feb 5 Eurosurveillance
Jan 29 CIDRAP News item on earlier Canadian estimate
Jan 15 CIDRAP News story on CDC VE estimate
Related Dec 5, 2014, CIDRAP News story