Why Some Kids Need a Second Dose Of Flu Vaccine
Dec 05, 2012

Recently we received this question on our Vaccinate Your Family Facebook page:

“My child has already received his first dose of flu vaccine over a month ago, but lately I’ve been wondering if it’s really necessary for him to get his second dose?  Is he protected at all from one dose?  And will the second dose be any different?”

The following response has been provided by Dr. Lara Zibners*:

DrZibnersAh, microbiology. As much as I think “The Killer T Cells” would be an awesome name for a band, I’m not going to bore you with a lecture on B-cells and macrophages, cytokines, and lymphocytes. Mostly because, as much as I loved microbiology back in school, my memory of the intricate ways in which our immune systems perform a complex and perfectly coordinated dance to protect us from infection is a bit rusty.
I’m an ER pediatrician, remember? My focus is on caring for children who are already ill. Keeping them from getting sick is almost counter-productive from a career stance, if you know what I mean.  Darn me to be so passionate about healthy, happy children; about the miracle of vaccination. Sigh. Anyway, let me see if I can’t shake a few files loose, dust them off, and answer your question in a way that even I can understand. We’ll call this, “How the immune system works, Dr. Zibners’ style.” Ready?
Okay. A vaccine is really a kind of “fake infection” that stimulates the immune system so that it can defend the body against a real infection later by creating antibodies specific to that bacteria or virus. These antibodies then float around, bored out of their skulls, I imagine, since they have only one purpose in life. But if said antibody is lucky, one day he bumps into the very “antigen” he was created for, sticks to him like cat hair to a black sweater, and signals the body’s attack system to come and remove the invader. All clear so far? Okay, moving on.
When I was 10, I didn’t know a word of French. (Stick with me. This is going somewhere.) When I was 11, Madame Wheaton appeared in my classroom and began pointing around the room, yelling “L’ecole” this and “Au revoir” that. I thought it was cool. And I was the class nerd. So I did all my homework and studied my vocabulary words and developed a rather basic mastery of the language. At school I was definitely at the top of my class when Madame was in the room. But imagine if I’d gotten on a plane and flown to Paris. How far do you think a few months of middle school French would have gotten me? Exactly.
The way some vaccines work is kind of the same. More than one dose is needed to give the best possible protection against infection. (Or in my case, about 3 more years of French before I could order a croissant without hearing any snickering.) This is true of many vaccines. It’s just a fact. It has to do with “priming” the immune system to mount a response and creating memory cells.  It also hinges on a whole host of other factors such as the age of the person, the design of the vaccine, and the body’s response to that type of infection. But that’s all boring and well beyond what we need to know to answer the question, so let’s leave that for another day, eh?
In other words, a kid younger than age 9 may not have suffered a true bout of influenza. If he also has never had a flu vaccination, then his immunity against influenza is about as strong as my French verb conjugations were in the 4th grade. Which means when it comes to the influenza vaccine, the first immunization may not result in high enough levels of antibodies. So a second dose is necessary to bring the circulating safety net to a level that will adequately protect a child.
Those of us age 9 and older have probably already encountered influenza at some point in our lives.  Therefore, even if we have never had a flu vaccine, our bodies probably have enough circulating “memory” against influenza.  In which case, a single dose will give us enough protection against infection if we were to encounter  one of the anticipated strains of influenza that is expected during the current or upcoming season.
Which brings up another point.

Why do those of us who have had a flu vaccine last year need one this year?

Because every year the vaccine is adjusted to include the specific strains of infection that very smart scientists have decided are the greatest risk to us. This year we’re looking at A/California/7/2009 (H1N1); A/Victoria/361-2011; B/Wisconsin/1/2010. Notice that we’re protecting ourselves from three different strains of influenza, one of them the dreaded H1N1 of 2009.
And that brings up yet one more point. Not only should a child have more than one influenza vaccine in his lifetime, but he also needs to have had at least one dose of an H1N1-containing vaccine, which was first available as a separate shot in 2009, then combined with our annual influenza vaccine since 2010.
So, to summarize, here is how we determine the recommendations for the 2012-2013 influenza immunization schedule for children 6 months through 8 years:

  • Has the child ever received an influenza vaccine? No or don’t know = vaccinate with two doses given at least 4 weeks apart.
  • Has the child received 2 or more doses since July 1, 2010? No or don’t know = vaccinate with two doses given at least 4 weeks apart.
  • Is the kid 9 years or older? Or is he younger than 9 but has documentation showing at least 2 prior vaccinations, at least one of which contained H1N1? Yes = one dose.

These recommendations apply whether a child has received the inactivated injection (approved for those 6 months and older) or the live-attenuated nasal spray (for kids over age 2 who are otherwise healthy and have no contraindications to the nasal spray).
Hopefully this has helped clarify the second dose requirement.  I know it’s inconvenient and miserable, dragging your child off for yet another dose of the flu vaccine. But a kid under age 9, who has only had one dose of flu vaccine, has sub-optimal, or even no real protection against influenza.  And we don’t want that, do we?
Oh? What’s that? How’s my French these days? Right. Um. So I sort of let it slide a bit after high school.  A while later I needed to learn German to communicate with my in-laws so if you ask me to how to say, “Bless you,” in French, I’ll probably slip up and say “Gesundheit.” But, on a recent trip to Paris I found that I could understand a few French words. As the week went by, my memory became clearer, the nouns and verbs first appeared as ghosts but then they took shape as real words passing through my lips.  But wait! That’s a conversation about booster shots. We’ll save that discussion for tetanus sometime soon, okay?

For more information on influenza vaccine recommendations, including who should and should not be vaccinated, head to these sites:


* Dr. Lara Zibners graduated cum laude from the Ohio State University School of Medicine. She completed both a residency in pediatrics and a fellowship in pediatric emergency medicine at Nationwide Children’s Hospital in Columbus, Ohio, and is currently board certified in both general pediatrics and pediatric emergency medicine. As the author of the award-winning “If Your Kid Eats This Book, Everything Will Still Be Okay,” and the hilarious blog we love at www.drzibners.com, she has been an avid and very public supporter of vaccination. When not speaking, writing or doctoring, Dr. Zibners does her best to mother two toddlers and a middle-aged husband, all of whom are fully vaccinated.

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