Does the influenza vaccine work? A small observational study

The Pediatric Insider

© 2014 Roy Benaroch, MD

Flu season is in full swing here, and I’m seeing dozens of feverish, miserable kids a week. Since it started early this year, maybe that means influenza will burn out and be over soon—but maybe not. Some years we get a “double dip” as a second strain of flu moves through town.

Influenza does a very good job of working its way through our communities each year. The symptoms of flu, including runny nose and cough, make transmission of infected mucus almost guaranteed.  And the virus itself, already very contagious, changes over time– so neither natural infection nor immunizations provide reliable lasting protection.

Though far from perfect, influenza vaccinations should be an important part of your family’s flu prevention strategy. Their effectiveness varies from year to year, but is probably overall in the range of 50-70%. Not great, but if even half of the cases of flu could be prevented, that’s a whole lot less misery, and far fewer people continuing to spread infection. Remember: for every case of influenza prevented, that’s fewer exposures for the rest of us.

I get asked a lot: how’s this year’s vaccine doing? Does it work? So a few days ago I collected data from my practice. I copied out the log book we keep of flu tests from 12-18-2013 to 12-24-2013—this is a list of all of the rapid flu tests we did in one of my two offices, the names of the patients and the results of the tests. Then I went back through their charts to see if they had been fully vaccinated against influenza this year.

Here’s the data, raw, in a “2×2 contingency table”:

	Vaccine Yes	Vaccine No	total
Flu test POS	2	14	16
Flu test NEG	9	9	18
Totals	11	23	34

I’m not going to go into big-time statistics with this—I’m not pretending that this was a full-scale, professional study. This was just a convenience sample of kids who had flu tests done in my office over a few days in December. But what it does show is striking:

• If you had a flu vaccine, your chance of testing positive for influenza was about 20%.
• If you didn’t have a flu vaccine, your chance of testing positive for flu was 60%– three times the risk.

For you statistics types, I did plug these numbers into a web-based statistics package, and based on the Chi-Square calculation for a 2×2 contingency table the difference was statistically significant with p<0.05. However: I’m not 100% sure I did that right. I’m not swearing by my statistical chops here. Anyone out there with a good statistics background: if you want to chew on the data, please do, and post in the comments what you’ve determined.

Now, to be honest, there are some big-time caveats to this “study.” We didn’t systematically test people based on certain criteria. Each doc decided who to test, and it’s possible that some of my docs would be more or less likely to test people depending on whether they had had a flu vaccine. The test itself isn’t perfect—though a positive test is quite reliable, it’s possible that a negative flu test misses up to even 50% of true flu cases. The study wasn’t a randomized clinical trial- whether or not each child was vaccinated was up to the parents, and could have been influenced by their individual child’s risks of influenza exposure. And I didn’t look at the timing of the vaccines that were given—it’s possible that some kids in the “Vaccine YES” column received their vaccines too late in the season to be effective, and should have been counted as “Vaccine NO.” Furthermore: I only had access to my own records. Some of the “Vaccine NO” children could have gotten flu vaccines elsewhere.

Still: from this small sample, it looks like flu vaccines were strongly effective at preventing influenza in children at my practice.

More on preventing and treating influenza

Explore posts in the same categories: Medical problems, Pediatric Insider information

Tags: flu, immunizations, influenza, vaccines

You can comment below, or link to this permanent URL from your own site.