Complete flu vaccination key to reducing flu risk in children

Children who are fully vaccinated against the flu, ie, receive the recommended number of influenza vaccine doses, have a lower risk of acquiring influenza compared with those who are partially vaccinated, a US study found.

“[W]e detected higher vaccine effectiveness (VE) against medically attended influenza in fully vaccinated children compared with partially vaccinated children,” said the researchers. “The risk of influenza was lower and VE was higher for young, previously unvaccinated children who received two doses in the current season vs one dose,” they added.

The researchers looked at data of 7,533 children aged 6 months to 8 years (65 percent <5 years, 46 percent female) who presented for outpatient care at five sites in the US Influenza Vaccine Effectiveness Network within 7 days of acute respiratory illness (with cough) during influenza seasons in 2014–2018 (four influenza seasons).

The children’s influenza vaccination history (one or two doses of inactivated vaccine) during the enrolment season was obtained from medical records. Sixty-eight percent had received initial vaccination prior to the current season, while 12 percent were first vaccinated in the current season; twenty percent were unvaccinated. Of the 5,995 who were vaccinated, 60 percent received two doses during their initial vaccination season.

In the current season, 39 percent were considered fully vaccinated*, though only 17 percent received two vaccine doses. Fifty-two percent of children were unvaccinated in the current season and 9 percent were partially vaccinated. Among children with any prior vaccination, VE was 51 and 41 percent among fully and partially vaccinated children, respectively, compared with unvaccinated children, with full vaccination status tied to a lower risk of developing influenza compared with partial vaccination (adjusted odds ratio [adjOR], 0.78, 95 percent confidence interval [CI], 0.61–1.01). [JAMA Pediatr 2020;doi:10.1001/jamapediatrics.2020.0372]

Among children recommended for two vaccine doses in the current season, VE was 58 percent vs 46 percent in fully vs partially vaccinated children, with a lower risk of influenza among those who received two doses vs one dose (adjOR, 0.61, 95 percent CI, 0.40–0.94).

In an analysis of 1,519 vaccine-naïve children aged 6 months to 2 years, children who received two doses of the vaccine in the current season showed greater VE compared with those who received one dose (53 percent vs 23 percent). Children who received two doses were less likely to acquire influenza than those who received one dose (adjOR, 0.57, 95 percent CI, 0.35–0.93).

“Compared with older children, young children, even if healthy, are at an elevated risk of influenza infection and influenza-associated complications, such as hospitalization,” said the researchers.

“The higher risk of infection resulting from underdeveloped immune and respiratory tract systems provides a reason to identify vaccination strategies focusing on this vulnerable population of younger children. Our study suggests a benefit of receiving two doses of inactivated influenza in the first season of vaccination as well as full vaccination as recommended by the ACIP**.”

The researchers noted that VE may differ according to type of vaccine (half or full dose). “It is [also] unknown how initial exposure to natural infection differs from priming with influenza vaccine,” they said.

“Promoting efforts to improve influenza vaccine coverage – particularly with two doses in the first vaccination season – may reduce the burden of influenza illness among young children, who are particularly vulnerable to complications and death from influenza infection,” they recommended.