What is pandemic H1N1?  How is it different from other influenzas?

There are three types of influenza A, B and C.  C is very rare and won’t be discussed further.  Influenza A and B can each infect humans.  Influenza A can also infect birds, pigs, horses, cats and dogs.  Sometimes influenza viruses cross between species, moving from pigs to humans or birds to humans.  That is where the terms swine flu or avian flu come into use.  They usually denote a virus that had its origin in another species and then moved to humans. Pandemic H1N1 virus is one type of influenza A virus.  There are other seasonal H1N1 viruses that have been in circulation for some time.

The influenza viruses have two proteins called hemagglutanin and neuraminidase which further help to distinguish one virus from another.  The current pandemic flu virus is H1N1.  The avian influenza virus that has been circulating among birds (and sometimes infecting humans) in Asia is H5N1.  The avian influenza virus causes severe disease in humans, but is not able to circulate easily from person to person.  It is not a seasonal flu strain.  Three types of influenza virus are usually in the seasonal vaccine: H1N1 (not the pandemic strain), H3N2 (another A virus) and a strain of influenza B virus.

There are differences between seasonal influenza and pandemic H1N1.  Seasonal influenza causes the greatest illness in older adults (64 years of age) and young children.  Most people that become severely ill with seasonal flu do so because they get a secondary infection.  Most people have some immunity to seasonal flu.  Pandemic H1N1 has behaved differently.  The greatest number of pandemic H1N1 infections thus far is in the age group 5 years to 24 years.  Pregnant women seem to be at higher risk to have a severe illness.  People that are severely ill with pandemic H1N1 usually have a pneumonia caused by influenza virus and not a secondary infection.  Essentially no one is immune to pandemic H1N1 virus.

What kind of influenza tests are there and what do they mean?

There are four types of tests that are being used to test for influenza.  The first is a rapid influenza diagnostic test (RIDT).  This test is a point-of-care test; that is, it can be performed in a clinic or doctor’s office.  There are several different RIDTs for influenza available.  They differ in sensitivity and specificity in detecting influenza viruses, and their ability to distinguish between influenza virus types (A versus B) and influenza A subtypes (e.g. novel H1N1 versus seasonal H1N1 versus seasonal H3N2 viruses).  So an RIDT result may be that the patient has influenza A or B, or that they have a sub-type of influenza A (seasonal or pandemic).  The problem with the RIDTs is that their sensitivity ranges from 40% to 69% for the pandemic H1N1 virus.  In other words, up to 60% of the time, a person can have influenza, but their test result on the RIDT will be negative.  Therefore, if a student has a negative “rapid test” in their doctor’s office, it does not rule out pandemic H1N1 or another strain of flu, and the student still needs to be restricted from school until they are 24 hours without fever.  For more information on RIDTs, see the August 6, 2009 MMWR article “Evaluation of Rapid Influenza Diagnostic Tests for Detection of Novel Influenza A (H1N1) Virus – United States, 2009”.

Another test which is often used for influenza diagnosis is the direct fluorescent antibody test (DFA).  DFAs are subjective, because the laboratorian has to look for a “degree” of fluorescence.  There is no quantitative number or direct comparison that is possible.

A third test which is used is rt-PCR (reverse transcriptase-polymerase chain reaction).  This test looks for influenza virus RNA and matches it to known genetic patterns.  The PCR test is the gold standard for influenza diagnosis.  It can detect influenza virus even when the viral load is not high.  It can also differentiate between influenza A and B, and different strains of influenza A.  This test takes several hours to run and is often a send-out, meaning that getting results takes a few days.

The final diagnostic test is a viral culture, where the laboratorian attempts to encourage the virus to grow in culture medium.  A positive result means the person has influenza, but a negative result does not necessarily mean a person does not have influenza, because there can be problems with specimen collection and transport that kill virus particles before they can be grown in the laboratory.  It takes several days to obtain the results of this test.

Most people will not need to be tested when they have an influenza-like illness (ILI).  Physicians will make decisions based on clinical judgment, because the more accurate tests take too long to give results.  This means, too, that decisions about whom to restrict from school will also have to be made based on clinical symptoms.

How long do ill children and staff have to stay out of school?

People who are ill with ILI (fever and cough or sore throat or runny nose) need to stay home for at least 24 hours after their last fever (without being on anti-pyretics).

Students and staff should isolate themselves at home.  They should only go to their doctor if they have underlying medical conditions, or if their symptoms worsen.  Testing is generally not necessary (or desirable). 

The exception to this 24 hour rule is people who attend or work in a setting where there are high numbers of individuals at high-risk of complications.  In that type of setting, ill people should stay home until 7 days after the symptoms begin, or until they have not had a fever for 24 hours (without medication), whichever is longer.  Settings with high numbers of high-risk people would include people in healthcare settings, children and staff in childcare settings for children under the age of 5, and attendees and staff in programs specifically for medically fragile populations.  For more information about which groups are considered high-risk with regard to pandemic H1N1, go to http://www.cdc.gov/h1n1flu/vaccination/acip.htm   This web page has recommendations for vaccination for pandemic H1N1 and lists the groups considered high risk.

For additional guidance for schools preparing for the flu, see the CDC guide “Preparing for the Flu (Including 2009 H1N1 Flu)” at http://www.cdc.gov/h1n1flu/schools/toolkit/ (scroll down to see toolkit).

What about the pandemic H1N1 vaccine?

There will be two influenza vaccines this fall – one for the seasonal influenza and one for pandemic H1N1.

The recommendations for seasonal influenza vaccine are the same as last year.  Children between 6 months and 18 years should be vaccinated.  The vaccine should become available in September.

The recommendations for pandemic H1N1 are slightly different, but children 6 months to 18 years should be vaccinated.  Everyone who gets the pandemic H1N1 vaccine will need two shots.  The vaccine will be available sometime later in the fall.  For more information about this vaccine, go to http://www.cdc.gov/h1n1flu/vaccination/acip.htm

Thus, children should receive two influenza vaccines this season.

School Nurse/Public Health Meeting
August 7, 2009