Let's get this out of the way up front: the evidence shows that influenza vaccines are effective in children between the ages 6 and 24 months. That's why they are recommended for children six months and up. I’ve seen myths circulating around the internet that the influenza vaccine isn’t effective in this age group, so I thought I’d try to put that to rest, as well as talk about why it is important that young children get two doses of influenza vaccine during their first influenza season.
Does Influenza Vaccine Work in Kids Under Two?
Yes. Let's get right to the data. The most recent study I'm aware of is a 2014 study in Pediatrics by Blyth et al, and not funded by any pharmaceutical company. It looked at influenza seasons in 2008 and 2010-2012 (it excluded 2009 as that year, there was the recommendation to receive the separate H1N1 vaccine), and found flu shots to be 86% effective in kids under two, concluding "TIV was effective in children aged <2 years. Despite demonstrated vaccine effectiveness, uptake of TIV remains suboptimal."
Further, from The Lancet, Heinonen et al 2011:
"Trivalent inactivated influenza vaccine was effective in preventing influenza in young children, including those younger than 2 years. Our findings suggest that influenza vaccine recommendations should be reassessed in most countries."
From Pediatrics again, Ritzwoller et al 2005:
"To our knowledge, this study is the largest study evaluating the effectiveness of influenza vaccine among children 6 to 23 months of age and the effectiveness of 1 vs 2 doses among children 6 months to 8 years of age. Our findings suggest that both doses are needed for previously unvaccinated children 6 months to 8 years of age, to provide maximal protection against influenza. In addition, despite the suboptimal antigenic match between the influenza vaccine and the circulating virus strain in the autumn of 2003, influenza vaccination was found to be effective in preventing medically attended ILIs among children 6 to 23 months of age, as well as all children 6 months to 8 years of age."
In Journal of the American Medical Association, Hoberman et al 2003, a two year study found the vaccine 60-69%% effective the year that influenza was endemic in the community but no effect, not too surprisingly, the year influenza was rare in the community.
"In the first cohort, efficacy rates against influenza in children aged 6 to 12 months, 13 to 18 months, and 19 to 24 months were 63%, 66%, and 69%, respectively."
Numerous other studies show immunogenicity of influenza vaccines under the age of 24 months, including as young as six weeks (though the vaccine isn't currently recommended under the age of six months). Examples are here, here, and here, and this is by no mean to be an exhaustive list.
Now, the reason you might hear a flu shot doubter claim the vaccine is ineffective in kids under two is because of language used in a 2012 Cochrane Review of influenza vaccine efficacy, which states, "Inactivated vaccines in children aged two years or younger are not significantly more efficacious than placebo."
How is that conclusion reached, given the above data? Well, meta-analyses like this one necessarily don't look at all the data, but only studies that meet certain parameters for inclusion. For whatever reasons, none of these were included in the review. The only study that was included in the review was from 1976, which looked at a whopping 16(!) infants, and used a single-strain, highly reactogenic influenza vaccine, a far cry from the trivalent and quadrivalent vaccines used today. This is completely irrelevant to modern policy-making, and no basis to declare the vaccine similar to placebo.
Cochrane Reviews are generally well-respected, and meta-analyses like these can be great sources for supporting a claim, but they can also reflect biases of the authors in terms of study selection and interpretation of the results. The lead author of this review has received such criticism.
Why Do Some Kids Get Two Doses?
One thing recognized from these studies that *one* dose of influenza vaccine is not effective in children under two years of age. This is why two doses of influenza vaccine, separated by a month, are recommended for children under the age of nine the first season they are vaccinated. The first dose primes the immune system, while the second dose actually provides the immunity. Because of similarities between flu shots from year to year, only one dose is needed each consecutive year after that.
This is why, if you have a young child, it’s worth getting your child’s second dose of influenza vaccine even if it is late in the season. For one, the second dose will help prevent influenza through the rest of the season, and influenza can circulate surprisingly late some years, sometimes well into spring.
Second, if your child gets their second dose this season, they will only need one dose next season, and it will be ready to work immediately when they get it. And since young children can be at the highest risk of complications from influenza, it’s absolutely worth making sure they are as protected as possible.