written by Tara Haelle

It’s been well established that the flu vaccine is one of our least effective vaccines, if not the least effective. As I’ve noted before, that low level of effectiveness is not a reason to skip getting the vaccine since some protection is better than none, and some seasons the flu vaccine’s protection is upwards of 70% effective.

What has been more problematic, however, is the limited evidence related to the flu vaccine’s effectiveness in young children. One Cochrane Review found little evidence of effectiveness for toddlers and infants and pointed out that many studies had been funded by pharmaceutical companies, though that analysis had problems of its own and other studies have shown effectiveness for the youngest kids.

Now, however, a new study unassociated with any pharmaceutical companies provides even better evidence of flu vaccine’s effectiveness in children. A study in Pediatrics today, funded by the Western Australia Department of Health, tracked 2,001 children for four years (2008-2012) and found the vaccine to be particularly effective for children under 2 years old – the same group for which we had the least data previously.

There are a number of strengths to this study aside from the funding source*. (Funding from pharmaceutical companies does not mean the data in a study is invalid, but I’m aware that many are less trusting of pharma-funded research.) For one thing, the study was prospective, which means the researchers followed children forward in time rather than asking their parents after the fact whether the children have been vaccinated and whether the children have experienced a flu-like illness.

Whether children were vaccinated was reported by parents but then confirmed through national immunization register or the children’s medical providers. Then, only children who tested positive for the flu were counted as having influenza, instead of any child experiencing a flulike illness. In fact, the vaccination rate of children who did catch the flu was compared to the rate and children were tested negative for the flu and then to children who tested positive for any other respiratory virus. Meanwhile, most previous studies counted any flu-like illness as the flu, which led to underestimates of flu vaccines’ effectiveness.

The study is also a reasonably good size with about 2,000 children, although the final analysis only included about 1,500 children because 2009 cases were excluded. That was the year there was a significant mismatch between the seasonal flu vaccine and the H1N1 strain, and this study looked only at seasonal flu vaccines, not including the separately administered H1N1 vaccine that year.

Among all 2,001 children, aged six months to four years old, 389 children, or one in five (20%), caught the flu while another 1,134 (60%) caught a different respiratory virus. Only 15.5% of the study participants were fully vaccinated, and 8% were partially vaccinated.

When the researchers compared children who caught the flu to children who tested negative for the flu for all children under 5 years old, they calculated the vaccine to be 65% effective. This effectiveness rate remained when they compared it to children who had other respiratory viruses besides the flu.

But the big news is for children under 2 years old: The vaccine was 86% effective for children aged 6 months to 23 months old. Just over half of those in the study, 806 children, were under 2 years old, so the study had a good-sized sample to work with for these findings as well.

Given the limited evidence we have had to date, this study offers especially good news. The only unfortunate news in the study was how poor the vaccine uptake was. Only a quarter of the kids had been vaccinated, and a third of these were only partially vaccinated. Children receiving the flu vaccine for the first time need to receive two doses, a month apart, to get the full protection afforded by the vaccine.

Once again, influenza is often not a mild disease, and not everyone recovers. Considering the vulnerability of children who catch influenza, including those who die, the flu vaccine, while not perfect, remains one of the best defenses available to protecting children from the flu.

*Four authors are members of the Vaccine Trials Group, Telethon Institute for Health Research, which has received funding for clinical trials from vaccine manufacturers, but none of this was associated with this study. One author has received past research funding from GlaxoSmithKline and CSL Ltd, and another leads a not-for-profit influenza-control organization that receives pharmaceutical company funding.