A reader submitted this comment to a recent PediaBlog post:

My five year old just had his yearly flu shot on Friday (as well as several other vaccines at his 5 year well visit).  We left the doc’s office around 10am and by 2pm he was complaining of a very sore throat.  When I checked his temp. a few hours later, he had one of about 100.5.  He didn’t complain of not feeling well at the doc appt. and when the doc did his well visit checkup, he didn’t mention anything not being right.  I called the nurse’s line to ask if him not feeling well/temperature would hinder the efficacy of the vaccines he received that day.  She said it was possible depending on the severity of his temperature.  What are your views?

We give influenza vaccine during cold and flu season, so we expect that some recipients will develop symptoms of illness shortly after the vaccine is given.  Four hours is a very short time from vaccine to illness and would suggest to a reasonable person that the vaccine was the cause.  However, multiple scientific studies have shown time and time again that there is nothing in an influenza shot that makes people sick.  The other vaccines received at the five-year check up also will not cause the symptoms you describe in your son.

An intranasal flu vaccine works differently than the injected, intramuscular flu shot.  FluMist is a “live” vaccine, meaning it has to cause a mild (almost always asymptomatic) infection in the recipient in order to create an immune response and be effective.  Such a vaccine could cause very mild symptoms.  Rarely, we will get a parental report of a child who gets a mild sore throat and stuffiness soon after receiving the FluMist.  Having experienced this once myself (I have received the intranasal flu vaccine for years), I can report that the side effect is feeling like you are coming down with something — you know, the mild stuffiness, scratchy throat, achiness that informs you that you are about to get sick with an upper respiratory illness (URI).  And that’s it.  That feeling lasts for 1-2 days and goes away, without actually getting a full-blown viral URI.

Our reader continues:

I’m guessing that he was developing a cold but didn’t have any symptoms at the time of the shots.  Do I need to worry that the shots aren’t going to do their job?  I’m prone to worrying so any feedback would be helpful.  P.S.:  His little sis had a temp. the day before, so we rescheduled her flu shot for this week.

Getting any vaccine when a person is suffering from a garden-variety viral or simple bacterial infection will not diminish the effectiveness of that vaccine (again, science tells us this).  However, kids feel bad enough when they are sick, so pediatricians prefer waiting until they are feeling better, rather than causing more misery (for most kids, ANY shot is misery!) while they’re sick.

I hope your son and his little sis are feeling better.  I agree that he was probably coming down with something coinciding with the timing of his check up, and may even have gotten it from his little sister.  Rescheduling her flu shot was the right thing to do.  Please call your pediatrician if their symptoms have not resolved.