Angela Campbell, MD, MPH: Vaccination in Respiratory-Ill Children

A new study led by investigators from the US Centers for Disease Control and Prevention (CDC) showed the influenza (flu) vaccine was associated with an overall halved risk of flu-related hospitalization among children with respiratory illness in the past 3 years.

The findings, presented at IDWeek 2019 in Washington, DC, show the vaccine’s recent efficacy in reducing pediatric hospitalization varies by flu strain.

In an interview with MD Magazine®, study author Angela P. Campbell, MD, MPH, medical officer for the CDC’s Influenza Division, explained the robust findings and what the varied outcomes implies for future research measures.

MD Mag: What was the intent of the pediatric influenza vaccine effectiveness trial among respiratory patients?

Campbell: Every year, we have estimates of how well the influenza vaccine works. And most of those large, national estimates come from our large outpatient network. So, the numbers that you see every season that hit the press are mostly numbers that say how well the vaccine prevented people from getting influenza illness that led them to a doctor visit.

But we're very interested in more in benefits against more severe outcomes of influenza. And so we have 2 networks now—one in adults and one in children—to look at how well the vaccine works to prevent hospitalizations from flu. And the new vaccine surveillance network, or NVSN, is the network that asks this question in children.

And we were very interested in this, because we have estimates from several years ago in the early 2000s but that at that time the NVSN was a three-site network, and now it's 7 academic centers—7 pediatric hospitals. So it's a much more robust network to ask these questions.

MD Mag: What were the key findings?

Campbell: The key finding really is that children who received influenza vaccine had a reduced risk of being hospitalized with flu. And more specifically, we looked over 2 influenza seasons—2016-17 and 2017-18. And in those seasons, children who received influenza vaccination, the likelihood of their being hospitalized with influenza was cut in half.

MD Mag: Why might vaccination be lower against the A(H3N2) virus?

Campbell: So we actually looked at protection against any influenza virus overall, and that protection was about 50% again, reducing the likelihood of being hospitalized in half. We also looked at it for specific viruses. So, for influenza A(H3N2) viruses, the vaccine effectiveness was 33%. For H1N1 viruses, it was 76%, and for influenza B viruses, it was in the 50% and 60% for each season.

And so this is not surprising to us. In many seasons and in our other networks, we do see that in H3N2 viruses, the effectiveness of our vaccine can often be lower against H3N2 viruses. There are likely multiple reasons for this.

Those viruses are more subject to change. They're also more subject to changes in the vaccine manufacturing process, which uses eggs. And the virus made for the vaccine is often a little different than the viruses that are circulating because of the production process. And this tends to affect the H3N2 viruses more than the other flu viruses.

So there are likely a number of reasons, but we do feel like our study highlights, yet again, the need to look and do further study into how we can improve effectiveness against all the viruses. But I would say, even 33% vaccine effectiveness means that it reduced the likelihood of hospitalization for those viruses by one-third, so that's still important.