Action Points  

• Explain that a single dose of an adjuvanted vaccine against the pandemic H1N1 flu was highly effective at preventing hospital admission for children with confirmed infection.
• Point out that the finding suggests that in an emergency in which vaccine is scarce, such as during a pandemic, children can benefit from a less than optimal immunization schedule.

A single dose of an adjuvanted vaccine against the pandemic H1N1 flu was highly effective at preventing hospital admission for children with confirmed infection, researchers reported.

In a case-control study conducted during the second wave of the 2009 pandemic, a single pediatric dose was 85% effective at keeping kids out of hospital, according to Rodica Gilca, MD, PhD, of the Institut National de Santé Publique du Québec in Québec city, and colleagues.

The finding suggests that in an emergency in which vaccine is scarce, such as the pandemic, children can benefit from a less than optimal immunization schedule, Gilca and colleagues argued online in Pediatrics.

“The decision to administer two doses of influenza vaccine to children is context dependent,” they concluded, with factors such as the stage of the epidemic and the availability of vaccine playing a role.

Young children are usually thought to be immunologically naive with respect to the flu, so a two-dose schedule is recommended for the first immunization with a conventional influenza vaccine lacking adjuvant.

That was the situation in the U.S. during the pandemic because American authorities have never approved an adjuvanted flu vaccine. but Canada used a vaccine boosted with an oil in water emulsion that demonstrated substantial immunogenicity in children at a single 0.25-milliliter dose, which contained 1.9 grams of hemagglutinin and half the adult dose of adjuvant.

To see how that adjuvanted vaccine played out, Gilca and colleagues conducted a case-control study starting Nov. 12, 2009 — 10 days after vaccination started in children in the province of Québec — and ended Dec. 19, when the second wave subsided.

Cases were children, ages 6 months to 9 years, with lab-confirmed pandemic flu who were admitted to hospital in the province. Controls, who were identified from health insurance records, were matched by date of birth and place of residence.

All told, the researchers analyzed 221 cases and 663 controls, with each case matched to three controls. the reference date for controls was the day symptoms appeared for the case child.

The primary endpoint was vaccine effectiveness, defined as one minus the odds ratio for hospital admission among vaccinated children, compared with those who were not vaccinated.

Gilca and colleagues found:

• Overall, a single pediatric dose of vaccine, given at least 14 days before onset of symptoms was 85% effective, with a 95% confidence interval from 61 to 94.
• Effectiveness varied with age — 92% in 6- through 23-month-olds, 89% in 2- through 4-year-olds, and 79% in 5- through 9-year-olds. the 95% confidence intervals were wide and overlapping at 51 to 99 for the first group, 34 to 98 for the second group, and -31 to 96 for the third group.
• Overall vaccine effectiveness for immunization no less than 10 days before symptoms appeared was slightly lower at 80%, and the age variation was similar.

The researchers cautioned that the study design meant they could not rule out residual confounding by unmeasured factors. As well, they noted, statistical power was limited because many cases occurred before the vaccine was available.

On the other hand, the study included all children admitted to hospital for the pandemic flu from a large population, and all cases were confirmed using real-time polymerase chain reaction testing.

The study was supported by the Quebec Ministry of Health.

Gilca reported financial links with Pfizer.