ATLANTA — Children aged 6 months through 8 years who did not receive at least one dose of a 2009 H1N1 influenza monovalent vaccine during the 2009–2010 influenza season should receive two doses of a 2010–2011 seasonal influenza vaccine, which will include H1N1 coverage, according to a new recommendation from the Centers for Disease Control and Prevention.
Prior recommendations stated that children in this age group who received two doses in the first influenza season in which they were vaccinated require only one dose the following season.
The recommendations by the CDC's Advisory Committee on Immunization Practices (ACIP) continue to state that children aged 6 months through 8 years who are receiving a seasonal vaccine for the first time should get two doses, and that those who received only one dose of a seasonal vaccine in the first influenza season in which they were vaccinated should receive two doses in the following season.
If children in this age group received at least one dose of 2009 H1N1 flu vaccine last year, they need only one dose of the seasonal vaccine in 2010–2011, provided they received two doses of seasonal flu vaccine in their first year of receiving seasonal vaccines.
The additional recommendation was made in light of immunogenicity and vaccine effectiveness data suggesting that children under age 9 years who receive only one dose of a 2009 H1N1 flu vaccine instead of the recommended two doses have reduced rates of seroconversion. Children in that age group also have lower rates of immunity resulting from natural influenza infection.
The ACIP Influenza Vaccine Working Group, which proposed the change, reported that protective titers of hemagglutinin inhibition following a single dose of a 2009 H1N1 monovalent vaccine during the 2009 pandemic occurred in as few as 19% of children aged 6 months to 3 years, and as few as 44% of those aged 3–9 years. Protective titers were present in at least 90% of older children and adults up to age 65 years, and in 81% of those aged 65 and older.
After two doses, 73%–100% of infants and young children developed protective titers.
Several other studies presented at the ACIP meeting also demonstrated the safety and efficacy of the 2009 H1N1 influenza monovalent vaccines.
Findings from a case-control study involving 4,156 individuals in four communities, and conducted from October 2009 through January 2010, showed that vaccine effectiveness for the prevention of confirmed medically attended 2009 H1N1 flu was 62% after adjusting for site, age, and onset date, Dr. David Shay of the CDC's influenza division reported.
Individuals in the study were considered vaccinated if they received at least one dose of vaccine more than 7 days before the onset of respiratory symptoms.
“Receipt of a U.S. monovalent, nonadjuvanted pandemic vaccine was associated with substantial protection against medically attended H1N1 illness,” he said, noting that vaccine efficacy was the same regardless of whether immunization occurred more than 7 days or more than 14 days prior to symptom onset.
As expected, the seasonal 2009–2010 flu vaccine had no effect on H1N1, he said.
As for H1N1 vaccine safety, data from several sources, including the CDC's Vaccine Safety Datalink (VSD), Post-Licensure Rapid Immunization Safety Monitoring (PRISM) project, Vaccine Adverse Event Reporting System (VAERS), and Emerging Infections Program Guillain-Barré Syndrome (GBS) surveillance project all showed that the H1N1 had comparable safety to seasonal vaccines.
The VSD and PRISM included data on 9 million individuals from eight health plans, and 35 million individuals from four health plans and nine state immunization registries, respectively, reported Dr. Tracy Lieu of Harvard Pilgrim Health Care Institute and Harvard Medical School, Boston.
The VSD data showed a possible link between the vaccine and GBS, and indicated a “weak signal” for Bell's palsy, but the larger PRISM network showed no indication of a link between the vaccine and GBS, Bell's palsy, or seizures.
Neither VSD nor PRISM showed any link between the H1N1 flu vaccine and up to 13 other possible outcomes being monitored, including pregnancy-related outcomes such as spontaneous abortion and preeclampsia.
Similarly, data from VAERS and the Emerging Infections Program indicated the H1N1 flu vaccine is relatively safe.
The VAERS data indicated no statistical signals for adverse events. Reported adverse events were consistent with expectations based on the literature; for example, GBS occurred in 1.1 per million doses, compared with an expected rate of 1.5 per million doses, reported Dr. Frank DeStefano of the CDC's immunization safety office.
Like VSD, the Emerging Infections Program showed a possible link between the vaccine and GBS, but as was reported in the Morbidity and Mortality Weekly Report in June (2010;59:657–61), the rate per 100,000 person-years was 1.92 in vaccinated individuals vs. 1.21 in nonvaccinated individuals for an age-adjusted rate ratio of 1.77.