Aug 12, 2004 (CIDRAP News) – Two studies from Colorado show that the 2003-04 influenza vaccine provided some protection even though it didn't match up well with the predominant flu virus, the Centers for Disease Control and Prevention (CDC) says.
One study showed that small children who had been fully vaccinated were 25% to 49% less likely than unvaccinated children to contract a flu-like illness, according to an article in the Aug 13 Morbidity and Mortality Weekly Report. The other study indicated that adults aged 50 to 64 years were 52% less likely to get the flu if they had been vaccinated.
"Influenza vaccine . . . can provide benefit even in years when the influenza vaccine and circulating strains are not matched optimally," the CDC report states.
To assess vaccine effectiveness in children, the investigators retrospectively compared rates of vaccination and medical treatment for influenza and related illnesses in 5,139 children enrolled in Kaiser Permanente Colorado. The study included children aged 6 to 23 months. Illnesses classified as either "influenza-like illness" (ILI) or "pneumonia and influenza" were counted as flu cases.
Children who had received two flu shots after September 2003 and at least 14 days before an illness were counted as fully vaccinated. Other children were counted as partially vaccinated or unvaccinated, depending on the number and timing of their shots. By Dec 7, 2003, 752 children were fully vaccinated and 1,388 were partially vaccinated.
When they compared fully vaccinated children with unvaccinated children, the investigators found that the vaccine was 25% effective against ILI (P=.052) and 49% effective against pneumonia and influenza (P=.022). However, the partially vaccinated children had no significant reduction in ILI or pneumonia and influenza. The report says the findings add to the evidence that children who have not been vaccinated before should receive two doses of flu vaccine for best protection.
To examine the vaccine's performance in adults, the investigators did a case-control study of people aged 50 to 64 years. The researchers used records from the Colorado Department of Public Health and Environment to identify people who had had a laboratory-confirmed case of influenza in November or December 2003. Controls who had not had flu in that period were recruited through random-digit phone dialing and were matched to patients by age, sex, and telephone area code.
The investigators found 304 patients and 1,055 controls who met the study criteria and completed telephone interviews. In the interviews, the participants were asked about flu vaccination and timing and other medical history matters. Half of the patients had medical conditions that increased their risk of flu-related complications.
The analysis showed that the vaccine was 52% effective in patients having no high-risk medical condition (95% confidence interval [CI], 31%-66%) and 38% effective in those with a high-risk condition (95% CI, 30%-60%).
The report notes that flu vaccine is expected to be about 70% to 90% effective when it is well matched with the major circulating virus strains. The CDC concludes that the 2003-04 vaccine was effective enough to justify recommendations to continue vaccination efforts when there is a "suboptimal match" between the vaccine and the circulating virus.
Last December, a preliminary study published by the CDC indicated that the flu vaccine was not effective in protecting Colorado healthcare workers from ILI. The new report says the preliminary study might have been too small to detect low effectiveness against ILI.
CDC. Assessment of the effectiveness of the 2003-04 influenza vaccine among children and adults—Colorado, 2003. MMWR 2004;53(31):707-10 [Full text]