H1N1 Vaccine: One dose or two?

The Pediatric Insider

© 2009 Roy Benaroch, MD

I seem to be spending a lot of time on H1N1 here, and a lot of time on H1N1 issues in my office. I can see myself, an old retired doctor in 50 years, sitting in my holo-rocking chair, listening to my octophonic MP6 player through my aural uplink (a new, never-released Michael Jackson tune!), reminiscing about the winter of 2009, the Swine Flu. “What’s a swine?” my grandchildren will ask.


For now, though, we’re in the thick of it. And some very smart people with brains larger than I are guessing that about a third of us in the USA are going to get this darned thing this winter. A third. Imagine that.


We still need to keep this in perspective. That’s a lot of sick people, but the vast majority of children and adults who get H1N1 flu recover fully in about five days. It’s uncomfortable and unpleasant and, well, pretty miserable for five days, but H1N1 has a very low rate of complications. Still, with perhaps 100 million Americans getting ill, even rare complications are going to occur more frequently than I want to see them. Some people are going to get quite ill, some people are going to be hospitalized, and some people are going to die. We should do what’s safe and effective to slow this epidemic.


How to prevent it? Stay away from sick people. Wash your hands. Don’t touch your face. Don’t go to work when you’re sick, and don’t send your kids to school when they’re sick. Workplaces should not encourage sick employees to work, and schools should not reward perfect attendance.


And please, get the dang vaccine. Forget the AM radio and internet nonsense: the government and the doctors do not want to kill you.


Getting all of this vaccine made and distributed has not been an easy task, and I wouldn’t call the government’s efforts one of their finest moments. At my office we’re depending on the folks at the Fulton County Health Department, who’ve so far sent us a tiny smidge of our expected order. With no advance warning, and no word on when we’ll get more. But the overall picture is improving—though the vaccine is dribbing out, it is getting out, and the trickle should become faster soon. Meanwhile, good studies continue to support the safety and effectiveness of immunization.


We’ve known for many years that children younger than 9 don’t seem to mount a strong immune response to flu vaccines, especially in their first year of immunization That’s why current recommendations for ordinary, seasonal flu vaccines suggest that if a child less than 9 is getting a flu vaccine for the first time, the dose should be repeated 28 days later.


It turns out that the swine flu vaccine is no different—and that’s no surprise, as it’s essentially the same vaccine that’s used to prevent influenza year after year, designed in this case to prevent this new H1N1 strain. The science, the development, the studies, and the factories are all the same as ordinary flu vaccines. Ignore hype that claims this vaccine is somehow more “new” than other flu vaccines.


So: it’s recommended by the ACIP (the advisory board on immunization practices of the CDC) that children less than 9 get two doses of the H1N1 vaccine this year for best protection. (Actually, some studies were done using a 10 year cut-off; so in some places the recommendation is up to age 10; but since that recommendation is up to 9 for seasonal flu, that’s the way most health departments and doctors’ offices are handling it.)


At the same time, we know that from a public health standpoint that the best way to put the brakes on the epidemic is to get as much of the population immune as quickly as possible. That will prevent the virus from spreading from person to person. Once the “herd” is mostly immune (or at least more immune), then all of us—vaccinated and unvaccinated—will be less likely to come in contact with anyone with flu, and therefore we’ll all be protected.


Though two doses are better than one, the CDC does not recommend that doctors “hoard” doses back to ensure that those second doses are given. We’ve been told—and it does make sense, when you think about it—that we ought to get as many kids vaccinated as possible with first doses, and continue vaccinating as long as we have vaccines, as quickly as possible. Since more and more doses are expected to be distributed in the coming weeks and months, it’s probable that second doses will be available, thought it can’t be guaranteed; and the timing might get tricky. Though a minimum of 28 days between doses is recommended, the interval can be longer than this. By the way, this recommendation is identical to what’s been recommended in previous years with ordinary, seasonal flu vaccines. Hoarding has never been encouraged by CDC or World Health Organization guidelines. Though two doses are better than one, one dose is far better than zero, and getting as many people as possible that one dose is going to help your child, and all of us, stuff this H1N1 genie back in the bottle.


Older H1N1 wisdom:

H1N1 vaccine after H1N1 illness?

A pandemic primer for parents

Flu: to shot, or not?


The CDC’s H1N1 site

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32 Comments on “H1N1 Vaccine: One dose or two?”

  1. Lara Lewis Says:

    I really appreciate all of your information concerning the H1N1. I have just one follow up question. You state that if a child under 9 is getting their flu shot for the first time in their life then they need to get a second dose. If my child received a seasonal flu shot last year, a seasonal flu shot this year and the first dose of the H1N1 this year does he need to get the second one? Will it hurt his body if he gets it and doesn’t need it?


  2. Jennifer Says:

    Dr. Roy,

    I just want to thank you for making the statement about children should not be rewarded for perfect attendance at school. That has always annoyed me because my kids stay home when they are sick and then when awards day arrives, they spend half of the awards ceremony handing out attendance awards and my kids don’t get one. I am a firm believer of keeping my kids at home when they are sick and I wish other people would do the same. After all, the kids are at school because the parent has sent them, so really, the parent should get the award…..


  3. Michael Says:

    I had read recently that the first dose was practically useless-as a side note I actually know two kids who had their first dose and contracted h1n1 several weeks later-is there any data to refute that?

    More importantly, are you aware of a maximum interval between injections before the first dose becomes ineffective? I’ve read that 28 days is preferred, while 21 days is acceptable. But my concern now with demand still outpacing supply is, how long is too long, if there is a “too long?”
    great post


  4. Dr. Roy Says:

    The studies looking at the American vaccine stratified children into groups 6 mos-5 years, 5-10 years, and 10 and up. After 1 dose of h1n1 injected vaccine, about 30% of the youngest kids (< 5 years) had good antibody responses; for the middle-aged 5-10 year olds is was about 60%, and for the kids 10 and up about 90% had protection. After a second dose, all children essentially had 90% protection. (I'm rounding off here)

    So I wouldn't say one dose is useless. 30% is better than nothing. That 30% decreases a child's risk of getting the illness, and also decreases the illness' severity, and also decreases the risk of spread to others. 60% is better than 30%, and 90% is better still; so if you can find a second dose, get it.

    It's certainly possible for a vaccinated child to still get H1N1 flu. The disease will probably be milder, but the vaccine is not perfect protection.

    There probably isn't a maximum interval between the doses, but that's based on studies of other flu strains seen from one winter to the next. H1N1 might behave differently.

    You might read that in other countries (eg Canada), only 1 dose is needed for children. Many other countries use an "adjuvant" to strengthen the immune response to vaccines. Adjuvanted vaccines can also be made more quickly, and more doses can be extracted from raw material. There are no adjuvants used the US flu vaccines, which makes them less immunogenic.


  5. Karen Says:

    Dr. Roy – Just curious why there are no adjuvants used in the US vaccines? Thank you. Karen


  6. Dr. Roy Says:

    Karen, in the US only one adjuvant is FDA approved– Alum. Though studies of adjuvant-containing flu vaccines were conducted, including one using Alum, there seemed to be a perception among American vaccine manufacturers that these vaccines would be greeted very skeptically by the public. Though not used in any American vaccines, squalene and other novel adjuvants have attracted the ire of anti-vaccine activists, and there are legal questions of whether purported vaccine injuries involving adjuvants could be the basis for a flood of lawsuits. There are only a handful of vaccine manufacturers in the US marketplace, and they’re staying away from anything possibly related to the manufactered, false “controversy” over the safety of vaccines.


  7. Chrissy Says:

    Fantastic! Thank you Dr. Roy! No drama…just the facts. Very much appreciated.


  8. kelly Says:

    I enjoyed your article and the follow up questions. I just heard from a concerned parent (I’m a nurse) that she was told that the vaccines are not effective if seasonal flu and H1N1 flu are given on the same day. I’m not talking about both laiv. This is the first I have heard of this. I’ve been giving seasonal flu and H1N1 at every visit if needed. any comments?


  9. Leah Says:

    How long in between doses before the vaccine is ineffective? My 8 month old baby had the H1n1 and Seasonal Flu Vaccine over 28 days ago and our pediatritian is out of the vaccines for round 2. Will my baby be protected now? We are flying this week and I was trying to get it all done before the trip.

    Your honesty is greatly appreciated.


  10. Dr. Roy Says:

    Leah, there’s no “too long” between doses. Though this hasn’t been specifically studied with the new H1N1 vaccine, with every other flu vaccine and every other vaccine, there is not an upper limit of time after which earlier vaccines don’t count. So get those boosters when they’re available, any time on or after 28 days past dose #1.


  11. Dr. Roy Says:


    You’re correct. Two LAIVs (“flumists”) should not be given on the same day, but two injected flu shots, or one injected and one mist, can be given simultaneously.

    Sorry it took so long to answer, your post got caught in the spam filter.


  12. Jessica Says:

    Dr. Roy-

    LOVE your insight! Great information! Thank you so much!

    I have a 7 month old baby boy and he has received one does of each type of flu vaccine (seasonal and H1N1) and now we are waiting for the 28 days to pass so he can hopefully get the second doses of each. It just seems like so many vaccines for such a little guy.


  13. Shanna Says:

    Dr. Roy,
    I am so sad that we’ve moved from Georgia! We miss you and Dr. Jennie! I don’t think we’ll ever find a pediatrician as special as either of the two of you. I have a question. I am still having trouble understanding if my girls will need a second dose of the H1N1 vaccine. They have received their first shot through the school system here in South Carolina. They have been getting the seasonal flu vaccine every year, and had 2 doses the first time they received the vaccine. If the H1N1 vaccine is so similar to the seasonal flu vaccine, will they still need to receive a second dose? If so, why?


  14. Elizabeth Says:

    Dr. Roy,

    Thank you so much for this great post. I work at the CDC, where mothers (with their young children in tow) have been protesting the H1N1 vaccine for weeks now (in the parking lot). It is beyond me that people think CDC exists for any other purpose than promoting the public’s health. Thank you for injecting (pun intended) some sanity into the absurd hype surrounding this vaccine.


  15. KarenK Says:

    Does the 2-dose recommendation for children apply only to the mist version, or the injectable as well? A recent newspaper article specifically mentioned the mist vaccine for a double-dose, so I wondered if the injectible has shown better immunity levels in kids after only one dose?


  16. Dr. Roy Says:

    KarenK, Both the mist and injectable H1N1 vaccines given before the 10th birthday ought to have a second dose given for best effectiveness. The immune responses to both vaccines in children are very similar.


  17. Dr. Roy Says:

    Shanna, the influenza vaccines are made the same way every year, with the same major ingredients. The only difference are the exact “antigens” that are in the mix– that’s the stuff that’s specific to each flu virus. This year, the new antigen is the H1N1 component. Next year, that same component will (likely) just be part of the ordinary seasonal vaccine, and won’t require a separate vaccine, but this year the H1N1 virus showed up too late to be included in the seasonal immunization, so a special separate vaccine was needed for this one missing antigen.

    Since the H1N1 antigen is so new, almost no one has ever been exposed to anything like it before (some elderly people do seem to have a close-enough antibody to have some protection, but certainly no children do.) Since it’s novel and new, a extra kick-start is needed for children to develop some protection, so a second dose is needed.


  18. Rob Says:

    My son is 7 and in the last 5 months, he’s had the “regular” Flu mist vaccine, he’s had H1N1 (office confirmed, but not CDC lab confirmed) and he’s had one H1N1 mist vaccine. Is it really a good idea to introduce a fourth exposure to the influenza virus, “reguar”, H1N1 or otherwise?
    Thanks in advance.


  19. Dr. Roy Says:

    Rob, you’re right in that it may not be necessary for your son to get an H1N1 mist booster, since he’s probably already had H1N1 flu. In most of the country now, H1N1 seems to be on the decline. On the other hand, there’s very little harm in your son getting another dose, and the CDC is warning us not to get too complacent– H1N1 may return as the weather gets colder. So I would say on balance, with the vaccine seeming to be so safe from all current reports, he probably should get the booster.


  20. Lin Says:

    My son’s birthday is Jan 1999, does he need 2nd dose of H1N1, is 2nd dose good for him?

    Thanks a lot.


  21. Dr. Roy Says:

    Lin, the exact recommendations for H1N1 are that children who have not yet reached their tenth birthday should receive two doses. Your son passed 10 in Jan 2009, so there is no need for him to get the vaccine.

    Just to confuse the picture: for the ordinary seasonal flu vaccine, the usual recommendation is that children who haven’t yet reached their ninth birthday should get two doses. I’m not sure why the H1N1 is up to 10th b’day, and seasonal is up to 9th, but that’s the way it’s supposed to be. The CDC however does say that clinicians can follow the recommendations for the seasonal flu for both kinds of vaccines, meaning that children who are past their ninth but haven’t yet reached their tenth birthdays are in a grey zone, depending on which guideline you follow. I’m not really happy with this vague recommendation, but that’s the whole story.


  22. Lin Says:

    Thanks, I asked because I thought following means protection for 10year old is between 60%-90%, but close to 60%,am I right?

    “for the middle-aged 5-10 year olds is was about 60%, and for the kids 10 and up about 90% had protection”


  23. Allison Says:

    Thanks for the great info on your site. Because the vaccine was so scarce in our area, I traveled to a neighboring state to get the first dose for my 2yo son. Now that the vaccine is finally available in our area, I’m hoping to get him the booster. Someone told me that the 1st and 2nd shots should come from the same “lot” to be effective…is that true? I have my doubts, and it would sure be more convenient to get it at our local pharmacy. Can you clarify?


  24. Dr. Roy Says:

    Allison, the doses certainly don’t have to come from the same lot, or even the same manufacturer. They can even be a mix of mist and injection. Get the second dose wherever it’s convenient for you.


  25. Karen Says:

    Hi Dr. Roy – First, thanks so much for answering all these questions – this is great! I’m back with a follow up question. My 7 1/2 year old son just got his 2nd dose of the h1n1 vaccine (nasal mist)on 12/16, and over the next two days, he has had a mild fever (100.9), a sore throat and chills, mostly at night. All the symptoms are gone now (12/18) and he wasn’t really that ill (he still played, ate, drank fluids) but definitely had something going on. Was this a side effect of the vaccine or did he just get h1n1? His pediatrician’s office said he was fairly well protected after the first dose (10/21), but advised us to get the 2nd dose if it became available at his school, which it did. He’s got a pretty strong immunity and the peake of h1n1 is down in his school, but I wondered how much protection the first dose gave him?


  26. Dr. Roy Says:

    Karen, the first dose of H1N1 vaccine probably offered ~ 60% protection, based on his age (the younger the child, the less protection one dose confers.) The mild symptoms your son experienced after vaccination could very well have been side effects from the vaccine, or an ordinary viral illness. I suppose it’s also possible that the brief illness was itself a mild case of H1N1 (made mild because of the partial protection of the single dose he had had.) In any case, I’m glad he’s feeling better.


  27. Karen Says:

    Thank you.


  28. Mrs Rachael Roberts Says:

    My 4 year old son was given the swine flu vacine yesterday and we had a call from our gp surgery last night telling us that there not sure if he was given the adults dosage or not.Feel very annoyed about this but they say their is nothing to worry about. How sure actually are they thats what is worrying me?


  29. Dr. Roy Says:

    Mrs. Roberts,

    I can tell from your email address that you hail from The United Kingdom. In the USA a four-year-old gets the same dose as an adult (actually, anyone who has reached their third birthday gets the “adult” dose.) This is the same dosing that’s used for the regular, seasonal flu vaccine that we’ve been distributing for many years. I don’t know how the product is dosed in the UK, though the comment you made that they don’t know whether he go the adult dose or not is kind of … odd. They ought to keep good records, and ought to know what they’re doing there. In any case, even if he was given twice the expected dose, the risks are still very small. Many countries use twice the strength of the US preparation to get a stronger response, and those kids do not have more serious side effects.


  30. Tina Says:

    I have 6 month old twin girls who have had the H1N1 shot. Following their shots at 6 months they both got fevers of 39 and one of them was completely miserable lethargic and not herself. I am contemplating whether to get her the second shot. What would be the consequences if I do not get the second shot for her.


  31. Dr. Roy Says:

    Tina, not getting the second shot will leave the girls less protected against H1N1. At this time, H1N1 cases have been on the decline in most parts of the country, so you’ll want to weigh the risks of their reaction against the risk of their contracting H1N1. I suggest you speak with your own doctor about their reaction and about the current, local prevalence of H1N1 in your area to help make the best decision.


  32. Sumeera Says:

    My baby is 9 months old i have given her one dose of h1n1 before 1 month is it necessary to give her sec dose


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