CDC: Imperfect flu vaccine was of help in tough season

Apr 17, 2008 (CIDRAP News) – Preliminary findings from Wisconsin suggest that this year's flu vaccine lowered one's risk of catching the flu by 44%, even though two of the three strains used in the vaccine didn't match well with the viruses in circulation, federal health officials announced today.

Forty-four percent is well below the 70% to 90% protection that a flu vaccine well-matched to circulating strains is expected to provide for healthy adults, but the Centers for Disease Control and Prevention (CDC) hailed the findings as good news in a year of "suboptimal" matches.

The CDC said the Wisconsin study indicated the vaccine provided 58% protection against influenza A/H3N2, the nation's predominant strain this season. However, the vaccine offered no protection against influenza B, the strain for which the vaccine was least well matched. The study provided no estimate of vaccine effectiveness against the third strain, A/H1N1, because no H1N1 viruses were identified in the Wisconsin patients.

"Although two of three vaccine strains were not optimally matched with circulating viruses this season, an interim VE [vaccine effectiveness] estimate suggests that vaccination provided substantial protection against medically attended acute respiratory illness in this study population," the CDC said in the Apr 18 issue of Morbidity and Mortality Weekly Report (MMWR).

The CDC also reported today that the flu season now ending was worse than the previous three and that flu vaccine production and distribution for the US market set records, though millions of doses again went unsold. The flu season was comparable with 2003-04, which was "moderately severe," officials said.

The six companies licensed to make flu vaccine for the US market produced about 140 million doses, about 20 million more than the previous season, said Dr. Jeanne Santoli, deputy director of immunization services in the CDC's National Center for Immunization and Respiratory Diseases.

"Based on data provided by distributors and manufacturers, we know that about 113 million doses of vaccine were distributed," Santoli said at a news teleconference. She said this set a record and was about 10 million more doses than were distributed in the 2006-07 season. The figures indicate that about 27 million doses of vaccine went unsold.

"We anticipate that next season's supply will be similar or somewhat increased from this season," Santoli said, adding that the CDC expects to have projections sometime in the next month.

Vaccine effectiveness study
The vaccine effectiveness findings came from a case-control study by the Marshfield (Wis.) Clinic. From Jan 21 to Mar 28, the clinic enrolled patients who sought care for an acute respiratory or febrile illness, testing them for flu and checking their vaccination status. The preliminary results are based on findings from 616 patients who were enrolled between Jan 21 and Feb 8. Patients were considered vaccinated if they had received a shot 2 weeks or more before enrollment or, for children younger than 9, if they had received two doses of vaccine.

Vaccine effectiveness was estimated by comparing patients who had laboratory-confirmed flu (as determined by reverse transcription polymerase chain reaction) with those who tested negative, according to the MMWR report. For patients who tested positive, samples were cultured to provide virus isolates for antigenic characterization.

Flu was confirmed in 191 of the 616 patients (31%), and 75% of the infections were type A. About 19% of patients who tested positive had received a flu shot, compared with 39% of those who tested negative.

As noted above, the findings yielded an overall vaccine effectiveness estimate of 44%. For H3N2 viruses—one of the subtypes for which this year's vaccine was not well matched—the estimated effectiveness was 58%. But for type B viruses, the other subtype marked by a suboptimal match this year, the vaccine's effectiveness was zero, the report says.

The study showed some differences by age-group: for healthy people aged 5 to 49 years, overall vaccine effectiveness was 54%, and effectiveness for H3N2 viruses was 68%. The report does not give figures for people aged 50 to 64 and over 65, who were small minorities.

Dr. Dan Jernigan, deputy director of the CDC's Influenza Division, said vaccine effectiveness estimates lower than 44% have been recorded in some years, most recently in 1997-98. He said those estimates came from studies using different methods than the Marshfield study.

"In the last 20 seasons, 16 have had good matches, and there have been four that had less than optimal matches," Jernigan said at the press conference. "In general the matches are better than what we have this year."

The MMWR article notes that the mismatch between vaccine and circulating viruses—in terms of both numbers and degree of antigenic difference—was sharpest for type B. About 95% of tested type B isolates this season belonged to a different lineage from the strain used in the vaccine. For H3N2 viruses, 71% of tested isolates were similar to A/Brisbane/10/2007, a recent variant of A/Wisconson/67/2005, the H3N2 strain used in the vaccine.

The degree of mismatch for H3N2 this season is "moderate" in comparison with antigenic differences seen in the long term for this subtype, the CDC says. In contrast, the B strain in the vaccine and the predominant circulating B strain are "substantially more antigenically distinct" than is the case with H3N2.

Although type A viruses—primarily H3N2—have been the most common type overall this season, type B viruses have made a late surge, becoming the most commonly reported type for the 2 weeks from Mar 23 to Apr 5, the CDC said in a separate MMWR article.

Given the vaccine mismatch for type B, the agency said, "If influenza B strains predominate during the remainder of this season, providers can anticipate an increased risk for vaccine failures and should consider early use of antiviral medications for treatment and prophylaxis of persons at high risk" for flu complications.

A fairly tough season
In describing the 2007-08 flu season so far, Jernigan said, "Most seasonal indicators have been on the high end compared with the past three seasons. . . . The 2007-08 season appears to be most similar to 2003-04, which was characterized as moderately severe."

This season, deaths attributed to pneumonia and influenza peaked at 9.1% of all deaths and stayed above the CDC's epidemic threshold for 13 straight weeks. In 2003-04, pneumonia and influenza deaths peaked at 10.4% of all deaths and hovered above the epidemic level for nine weeks.

Sixty-six children have died of lab-confirmed flu and its complications so far this season, Jernigan reported. Of those old enough for vaccination, 52 either were not vaccinated or did not receive both of the two recommended doses, he said.

For the week that ended Apr 5, six states still had widespread flu activity, and 11 states had regional activity, the CDC said.

CDC. Interim within-season estimate of the effectiveness of trivalent inactivated influenza vaccine—Marshfield, Wisconsin, 2007-08 influenza season. MMWR 2008 Apr 18;57(15):393-8 [Full text]

CDC. Update: influenza activity—September 30, 2007–April 5, 2008, and composition of the 2008-09 influenza vaccine. MMWR 2008 Apr 18;57(15):404-9 [Full text]

See also:

Apr 24, 2007, CIDRAP News story "Record flu vaccine supply expected for next season"

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