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Influenza ("The Flu")


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(with updates on swine flu,
the 2010-2011 and 2011-2012 flu vaccines,
the new intranasal flu vaccine,
antibiotic resistance -- and why you shouldn't just go out and take flu antibiotics,
and thimerosal-containing vaccines.

Influenza is a very common disease -- so common, in fact, that whenever we have the (classic) symptoms of fever, chills, headache, aches and pains, and "malaise" (the technical term for "feeling lousy") we tell ourselves and others that we have "the flu". This may be true, and often is true during the flu season (1-3 months during the winter), but you should bear in mind that there are many other viruses that can produce similar symptoms -- and a few bacteria as well.

After the initial "flu-like" symptoms, cough, congestion, and sore throat become worse in most cases; you may also have viral "pink eye" and/or abdominal symptoms (stomach ache, nausea, and vomiting). Muscle aches and pains can be bad enough to make walking difficult. Usually the symptoms subside within a week or so.

In small children, influenza can also cause bronchiolitis (inflammation of the small air passages of the lungs). The symptoms are similar to those seen with respiratory syncytial virus (RSV), and the treatment is similar. There are many other potential complications of influenza infection; one I see rather often during the season is temporary suppression of white blood cell production, which can make a child more susceptible to some bacterial infections.

There are three types of true influenza virus: A, B, and C. A and B are responsible for most of the flu epidemics we encounter. The A type can be divided into "strains" based on two antigens, the H and the N; every year or so at least one minor change occurs in either the H or N or both antigens, but that change is enough to make people who had the previously popular strain susceptible to the new one. Large changes in the antigen ("shift") of the A virus happen about every 10 years, and result in large epidemics, or pandemics, since most people aren't at all immune to the shifted virus. The most infamous flu pandemic was in 1918, near the end of the First World War: 20,000,000 people died of the flu -- more than were killed by World War I. The annual minor changes, or "drifts", may result in strains that lots of people are susceptible to, but this can take several years.

Each year the U.S. Centers for Disease Control and Prevention (CDC), the World Health Organization, and vaccine manufacturers try to predict which strains are most likely to cause flu epidemics for the next year, and then produce vaccines tailored to those strains. Because of the constant change in the flu virus, the vaccine is often changed each year to protect against the flu strains that are expected to be responsible for that year's flu infections -- and you need to get the new vaccine each year to be protected against that year's expected strains. The flu vaccine for a given season usually contains antigens from three different strains, two A strains and one B strain. For the 2009-2010 season, the strains were:

For the 2010-2011 and 2011-2012 seasons, the strains are:

The recommendations for a particular season are usually issued in the month of August preceding the season, or earlier. As of today, the 2012-2013 season's strains have not been confirmed. (As usual, I will post the 2012-2013 strain information as soon as I can after it is released.)

A strain is named based on its type (A, B, or C), where it was first isolated (it used to be country-based, but now isolations can be localized to a particular city), the year of isolation, and the strain number -- the A/California strain was isolated in 2009), and the "serial number" of the isolation for that year.

Influenza viruses actually infect wild and domestic birds, swine, and other animals, which makes them almost impossible to eradicate -- especially since most birds, including wild birds, can carry the viruses without getting sick. Some of the antigens in swine, avian (bird), and human influenza can trade places with each other and produce new and different strains of viruses. Swine flu, like human flu, is most common during the fall and winter. However, the flu viruses are always present in some swine (as they are in some people) in the off-season.

In 2009 there was an outbreak of an H1N1 flu, initially thought to be mainly a swine flu, which eventually spread to people worldwide. This H1N1 flu strain actually turned out to be very similar to the 1918 "Spanish" flu strain that killed more people than did World War I. The strain had been common from then until the mid-1950's, when pandemics (worldwide epidemics) of flu strains from Asia became more common. People who were born before the late 1950's were not as badly affected as children and younger adults, probably because they had some residual immunity from previous exposure, However, many young people, and some older people with other medical problems as well, died during the "swine flu" pandemic. I have more information on the swine flu outbreak at a separate page about swine flu.

Bird Flu

The "swine flu" of the mid-1970's was actually reported first in swine, and millions of Chinese chickens were destroyed in late 1998 to help control the possible spread of the Hong Kong flu strain to people. The currently circulating "bird flus" has killed some poultry; only a relatively few people have been infected with bird flu, and only about 100 people worldwide have died of it -- and all of them came in contact with infected birds. No one has yet gotten bird flu from another person (at least as of this writing.) We do not yet know if the bird flu virus could mutate into something that could be passed from person to person.

For that matter, we haven't really seen a single antigen pattern to bird flu. Most of the reported human infections were with an H5N1 flu virus, and the H5N1 strain is the most common bird flu strain and is often referred to as "the bird flu". There is a vaccine available for an H5N1 flu virus strain: the vaccine is not available because it has been stockpiled by the US government for use in an emergency. However, according to CDC, there have also been confirmed "bird flu" cases invloving H7N2, H7N3, H7N7, and H9N2 flu viruses. Since the vaccines have to be different for different strains, this could be a problem in case of an epidemic if a flu virus spreads faster than we can produce and distribute a vaccine against. Fortunately that hasn't happened yet, and our experience with the swine flu vaccine suggests that vaccine for a new strain of flu can be produced and distributed fairly quickly in an emergency -- although it will take at least some time to manufacture.

Who Should Receive Flu Vaccine?

In the past we usually did not vaccinate every child routinely against flu. However, because small children (between age 6 months and age 23 months) are more likely to land in the hospital if they get the flu, we encourage those children to receive the vaccine, and the Advisory Committee on Immunization Practices (which is responsible for recommending immunizations in the US) now recommends that everyone over age 6 months receive the flu vaccine.

Some children are likely to have severe problems if they get the flu, and they must be vaccinated every year. These include children with:

There are also some people who cannot receive flu vaccine. The most frequent reason not to get the vaccine is allergy to eggs: the vaccine viruses are "grown" on eggs, and some of the egg protein is still present in the vaccine. Your doctor can and will review all the potential reasons not to receive flu vaccine before giving it to you or your child.

The flu vaccine is usually given as a single dose. Children under 9 years old who are getting their first flu shot need two shots (4 weeks apart if the injectable vaccine is used, 6 weeks apart for the nasal-spray vaccine. If a child does not receive 2 doses of flu vaccine the first year he receives vaccine, he needs 2 doses the next year. After that, only one shot is needed each year (although all of this may change in a given year depending on the latest virus drifts and shifts).

It takes about a month after the vaccine is given to develop full immunity. Side effects usually include fever and aches and pains at the shot site. The outbreak of Guillain-Barre syndrome associated with the 1976-77 "swine" flu vaccine has not been seen again; Guillain-Barre is very rare among immunized people and has not been firmly connected to flu shots in kids.

Flu Antibiotics and Resistance

Unlike almost all viruses, there are antiviral antibiotics which can block "growth" (actually reproduction) of influenza viruses. They are most helpful very early in a case of flu; once you have a few million of the bugs in you the antivirals don't do much good.

There are four antiflu antivirals available at present. Two of these -- amantidine and rimantadine -- prevent the virus particles from taking off their outer coats (really!). Since the virus can't infect a cell without shedding the outer coat, it can't reproduce. These medicines work only on influenza A viruses. The other two -- zanamivir and oseltamivir -- block an enzyme called neuramidase (the N in the antigen code, by the way)on the surface of the virus, which reduces the number of new virus particles released by an infected cell. Since both influenza A and influenza B have neuramidase on their surface (they have to, to do their thing) both A and B can be blocked by zanamivir or oseltamivir.

All of these medicines, when properly used within the first 48 hours of illness, can reduce the severity of flu inefction, as well as how long you stay infected (by about 24 hours, but every little bit helps). They will also lessen the chance that someone else may get the flu from you. Three of these medicines (the odd one out is zanamivir, which has to be inhaled -- the others are pills or capsules that you swallow) have been shown to block transmission of the flu from person to person. This would be very helpful if we indeed run into a flu pandemic.

However, in past years more than 90% of flu viruses checked by CDC have been resistant to amantadine and rimantdine. This season's H3N2 flu virus seems to be treatable with amantadine, rimantdine, and with zanamivir as well, but 98% of H1H1 influenza A virus samples tested are resistant to oseltamivir. However, some isolated samples of the 2009 swine flu virus were resistant to oseltamvir, but not to zanamir. The H3N2 influenza virus, and the current influenza B virus, are susceptible to all four antivirals.

Why did this happen? I can think of two reasons:

Bugs are (sometimes) smart.
They may not be able to think, but they do mutate, and the mutations that can survive an antibiotic will survive and multiply. (The same thing happened with HIV and with most bacteria.)
We aren't that smart -- especially when it comes to using antibiotics properly.
Just like with antibacterial antibiotics, the more often we use antivirals improperly, especially in people who do not need them, the sooner we will see resistant viruses that we can't treat. Also, if you are given an antiviral and do not take the whole prescription just as it was prescribed, you increase your chances of having a resistant virus.
CDC recommended in 2007 that antiflu antivirals be used to prevent flu only by certain groups of people who were not vaccinated until after an outbreak of flu began in their area. These include:

Antivirals should be used only if they are likely to work with a particular patient. Practically speaking, this means that doctors need to know which flu strains are most "popular" in their area and select medicines accordingly. If possible, doctors should try to determine which flu strain a patient has and prescribe antivirals according to the results. For more information, see the CDC Health Advisory on Interim Recommendations for the Use of Influenza Antiviral Medications.

Thimerosal and Flu Vaccines

Some vaccines, including some of the available flu vaccines, contain thimerosal, a preservative used in many vaccines and other medicines (including contact-lens storage and cleaning solutions) to prevent bacteria from growing in the solutions. Until a few years ago, many vaccines contained thimerosal, but it has been removed from almost all of the vaccines we now give to children. The only widely-used vaccine that still contains thimerosal is the flu vaccine, and only some manufacturers still produce flu vaccine that contains thimerosal.

Thimerosal is a mercury-based chemical. There is a theoretical risk of mercury poisoning from the thimerosal used to preserve vaccines. Although several researchers claim that there is evidence of such poisoning, and in particular an increased risk of autism with thimerosal-containing vaccines, this has not been well-proven, and several large studies in different countries show no change in the occurence of autism or other neurodevelopmental disorders between children given thimerosal-containing and thimerosal-free vaccines. A study by California public-health authorities published in the January, 2008 issue of the Archives of General Psychiatry and quoted in the New York Times, found that the autism rate in children rose steadily from 1995 to 2007 even though thimerosal was removed from all vaccines, except for some flu vaccines, in 2001. If thimerosal was in fact responsible for autism, autism rates should have fallen after 2004. They didn't.

Influenza infection is dangerous to infants. However, only two of the three flu vaccines on the market are meant to be given to children under age 2 years, and those vaccines are not recommended for children younger than six months. Therefore, most public health authorities, including the CDC, recommend that infants at risk receive flu vaccine starting at age 6 months whether or not the vaccine contains thimerosal. Some of the current vaccine is made with a reduced amount of thimerosal, reducing the theoretical risk of mercury poisoning, and that vaccine should be used for vaccinating children under age 3 years.

I am aware that there are many people who believe that thimerosal should not be used to preserve vaccines. I believe that thimerosal should not be used to preserve vaccines if at all possible. I also know several people with autism and Asperger's syndrome, including one in my own family. However, I also believe that flu is sufficiently dangerous to patients, especially the very young (many of whom I admit to the hospital every year with complications of flu), that any risk from the preservative is MUCH less than that from influenza itself, and I would not hesitate to vaccinate my family against the flu even if I could not obtain preservative-free vaccine. (That is my opinion. Other people's opinions differ. Some people have also pointed out that in a flu pandemic, such as the 2009 swine flu outbreak, any new vaccine that is produced for the emergency will have to have thimerosal as a preservative, since there won't be time to produce single-dose vials of preservative-free vaccine. As always, you need to talk to your own or your child'd doctor to help decide if you or your child should receive the vaccine.)

The Nasal-Spray Flu Vaccine

We now have available a flu vaccine that can be sprayed into a patient's nose, rather than being given by injection. Flu-Mist®, or "live, attenuated, intranasal vaccine" (or "LAIV") is a solution of flu virus particles that are still alive but have been weakened and that grow best at temperatures way below that of the human body. Although the virus is alive, it is so weak that it will produce few, if any, symptoms in healthy people.

LAIV has been approved by the Food and Drug Administration for use in people age 2 years to 49 years. (The expansion to children 2-5 years old is new this season.) For several reasons, including the possibility of flu symptoms appearing, LAIV is not approved for use in children under 2 years, adults 50 or older, people with chronic diseases (like the ones listed above), or children receiving aspirin; these people have to receive the injectable vaccine. Also, a study published in the Journal of the American Medical Association on March 2, 2009 suggests that the nasal-spray vaccine is not as effective in adults as is the injectable vaccine.

As with the injectable vaccine, a child less than 9 years old needs 2 doses of the vaccine for full immunity in the first year she receives the vaccine (or 2 doses in the second year of vaccination if she only received one dose the first year), but the two doses of LAIV must be given 6 weeks apart rather than the 4 week separation for the injectable vaccine.

Supplies of Flu Vaccine

The injectable flu vaccine contains split particles of the flu virus, which can induce immunity but can't actually make you sick. However, the virus stock has to be grown before vaccine can be made from it, and growing the virus takes a lot of time. The CDC has to figure out what flu virus strains are needed for a given flu season 6-9 months before the season starts so that the manufacturers have enough time to produce vaccine.

There were many problems with flu vaccine supply in the last 3-4 years, resulting in real and practical shortages of flu vaccines in many areas. The supply system was further strained in 2009 by the need to produce a separate vaccine for the H1N1 "swine" flu strain. This is not a problem in 2010, since manufacturers had access to the prevalent strains of flu virus -- including the H1N1 "swine" strain -- well in advance. The vaccine manufacturers' performance in producing large amounts of vaccine for the H1N1 strain in 2009 on top of the year's regular flu vaccine production was rather remarkable, and likely helped slow down the spread of the H1N1 strain.

For more information on influenza, see the CDC Web site or the WHO Web site.


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PLEASE NOTE: As with all of this Web site, I try to give general answers to common questions my patients and their parents ask me in my (real) office. If you have specific questions about your child you must ask your child's regular doctor. No doctor can give completely accurate advice about a particular child without knowing and examining that child. I will be happy to try and answer general questions about children's health, but unless your child is a regular patient of mine I cannot give you specific advice.


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Copyright © 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Vinay N. Reddy, M.D. All rights reserved.
Written 09/02/96; major revision (information on swine flu) 04/25/09; swine flu information moved to swineflu.html on 04/29/09; last revised 05/23/12 counter