Canadian researchers today released a preliminary estimate that this year's influenza vaccine reduces the risk of infection from the 2009 pandemic H1N1 (pH1N1) virus by 74%—apparent good news in a season when that strain is by far the most common one in North America.
Writing in Eurosurveillance, the researchers also said the pH1N1 virus has changed little since it emerged nearly 5 years ago. That means the strain in the vaccine remains well matched to the circulating virus, and it should allay concern that evolution of the virus may account for its predominance this winter, they say.
"Our interim findings indicate that the 2013/14 TIV [trivalent influenza vaccine] provides substantial protection against resurgent but conserved A(H1N1)pdm09 viruses circulating in Canada during the 2013/14 season, reducing the risk of medically-attended laboratory-confirmed A(H1N1)pdm09 illness by about three quarters," says the report by Danuta Skowronski, MD, of the British Columbia Centre for Disease Control, and colleagues.
A more typical level of effectiveness for seasonal flu vaccines is around 60%. For example, a midseason analysis by the US Centers for Disease Control and Prevention in January 2013 found that the seasonal vaccine then was about 62% effective. Also, a careful meta-analysis of flu vaccine effectiveness (VE) studies published by researchers at the University of Minnesota's Center for Infectious Disease Research and Policy, publisher of CIDRAP News, in 2011 found that flu vaccines yielded about 59% protection in working-age adults.
The Canadian authors caution that their estimate is preliminary, based on only part of the season, and may change by its end.
A pH1N1 season
More than 90% of flu viruses that have been subtyped in both Canada and the United States this winter have been pH1N1 strains, the researchers note. That contrasts sharply with the same period last year, when 90% of isolates belonged to the H3N2 subtype, they say.
The team used the test-negative case-control method to assess VE, in which patients who have flu-like symptoms are tested for flu and their vaccination status is determined. Those who test positive for the virus are cases and those who test negative serve as controls.
The analysis covers the period from Nov 1, 2013, to Jan 23, 2014. Specimens were collected from 1,091 patients at sentinel clinics during that time, and 792 specimens were included in the primary analysis after some were excluded for reasons such as lack of data on vaccination timing. The specimens were tested using polymerase chain reaction (PCR).
Of the 792 samples, 325 (41%) were positive for flu, and 287 of 318 isolates that were subtyped were found to be pH1N1. Forty-one of 332 cases (12%) and 155 of 487 controls (32%) reported receiving this year's flu vaccine. After the researchers excluded some in each group because of problems with vaccination timing, 10% of cases and 29% of controls were considered immunized.
The adjusted estimate of effectiveness against pH1N1 was 74% (95% confidence interval [CI], 58%-83%), the authors report. They estimated VE against any flu strain at a slightly lower 71% (95% CI, 54%-81%).
The report also notes that all 473 pH1N1 isolates collected in Canada during the first 4 weeks of this flu season were antigenically similar to the 2009 California strain used in the vaccine, despite a change from clade 6C to clade 6B since last season. The article also describes other findings that show little change in the circulating pH1N1 strains.
The researchers comment that the strong resurgence of pH1N1 this winter after only low-level circulation the past few years has raised questions about possible virus evolution and reduced VE. "Our interim 2013/14 virological and VE analysis provides timely reassurance against both of these concerns," they write.
The finding of 74% VE is in "the upper range" of VE estimates for non-adjuvanted TIV against pH1N1 reported since 2010 in Canada, Europe, and the United States, which range from about 60% to 80%, the researchers say. During the 2009 pandemic, an adjuvanted monovalent pH1N1 vaccine used in Canada was estimated to be 93% effective, they note.
They comment that the pH1N1 virus's genetic stability is somewhat surprising, given that it has been circulating globally since 2009. However, H1N1 viruses historically have shown a slower pace of antigenic change than H3N2 viruses have, they add.
In other observations, the scientists say most of the study participants who were vaccinated this season also reported being immunized last season. They note that some recent studies have shown a trend toward higher VE with recurrent receipt of the pH1N1 vaccine, whereas other recent studies have reported the opposite.
Finding is not unexpected
Edward Belongia, MD, director of the Epidemiology Research Center at the Marshfield Clinic Research Foundation in Wisconsin, said the finding of 74% VE against pH1N1 is not surprising. "What they're reporting today is certainly in the ballpark of what we'd expect for a non-adjuvanted vaccine and the H1N1 virus," he told CIDRAP News.
For comparison, Belongia said the monovalent pH1N1 vaccine used in the United States during the 2009 pandemic was about 56% effective, and the 2010-11 seasonal flu vaccine showed a VE of 66% against pH1N1.
In recent years, he said, flu vaccines have tended to be more effective against H1N1 than other subtypes: "Confidence intervals can be wide, but we're certainly seeing a pattern where VE against H3N2 is not as impressive as we see for H1N1 or B. This is an H1N1 season, and last season was an H3N2 season. It's hard to do a direct comparison between last season and this one, because we're seeing very different viruses circulating."
Belongia called the study well done, adding, "I laud them for being able to pull it together very quickly and put it out early in the season."
No preliminary estimate of flu VE in the United States has been released yet this season. Belongia said he is part of a group that will be reporting preliminary VE findings next week, but he declined to comment on how the US results compare with the Canadian findings.
Skowronski DM, Chambers C, Sabaiduc S, et al. Interim estimates of 2013/14 vaccine effectiveness against influenza A(H1Na)pdm09 from Canada's sentinel surveillance network, January 2014. Eurosurveillance 2014 Feb 6;19(5) [Full text]
Jan 11, 2013, CIDRAP News story on CDC VE estimate last season
Oct 25, 2011, CIDRAP News story on meta-analysis of flu VE studies