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Influenza 2009 A(H1N1) ("Swine Flu")

including an update on the recall of some H1N1 vaccine for children

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(Note: this is a spinoff of my Web page on influenza in response to the current outbreak of "swine flu". I am updating frequently with information from the US Centers for Disease Control and Prevention, the World Health Organization, and the news media. This separate page discusses only "swine flu"; for more information on influenza in general, please see my main influenza page.

Influenza viruses actually infect wild and domestic birds, swine (pigs), 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.

According to estimates from the US Centers for Disease Control and Prevention, between 34,000,000 and 67,000,000 cases of humans infected with an H1N1 influenza A virus occurred in the US between April 1 and November 14, 2009, with cases confirmed in all 50 states, the District of Columbia, Puerto Rico, and the US Virgin Islands. Between 154,000 and 303,000 patients were admitted for problems related to the H1H1 strain and between 7,070 and 13,930 deaths have been attributed to this flu strain. The CDC stopped reporting numbers of cases and deaths on a state-by-state basis in late July and is now reporting numbers by US Department of Health and Human Services regions. As of December 26, only 4 states (Delaware, Maine, New Jersey, and Virginia) reported "widespread" flu activity, down from 7 states the previous week. 13 states (Alabama, Alaska, California, Connecticut, Florida, Georgia, Hawaii, Indiana, Kentucky, Nevada, New Mexico, New York, and Tennessee) have reported "regional" flu activity (flu cases limited only to certain areas of each state), and 19 states, as well as the District of Columbia, have reported flu activity limited to localised areas. The map below shows the progression of swine flu cases in the United States from the earliest cases at the end of April to early June, when cases were reported in every US state for the first time (darker colours correspond to higher numbers of confirmed cases).

Swine Flu Map

As of December 26, 2009, the Michigan Department of Community Health reported 692,986 cases of flu-like illness in Michigan, compared with 416,972 cases in all of 2008 and 404,476 in all of 2007. There have been 2006 hospital admissions and 64 deaths associated with influenza of any type since September 1, 2009 in Michigan.

Apparently the new strain of flu virus contains genetic material from swine, bird, and human flu viruses found in North America, as well as swine flu viruses found in Europe and Asia. Thus, this virus has been popularly referred to as "swine flu", although there has been only one report of pigs being infected with this strain (in Canada, in a pig exposed to a farm worker who had recently been in Mexico. Remember that influenza can be spread from one person or animal to another without physical contact -- sneezing can be enough). Most people who have died from complications of this flu strain in the US had other medical problems which may have made them more susceptible to influenza. This flu outbreak seems to be affecting mostly children and young adults, while seasonal flu usually hits the elderly hardest, and most reported cases have been mild (the CDC suspects that most people with the "swine flu" are not being tested because their symptoms are so mild, and that there may actually be over 100,000 cases in the US).

As of December 30, 2009, the World Health Organization (WHO) has reported at least 12220 deaths worldwide, with cases in over 208 countries. Central and Eastern Europe have had the most active recent flu activity, while activity has been declining in other parts of the world including North America. On June 11, WHO declared a phase 6 alert (sustained outbreaks in communities in two regions of the world, which constitutes a global pandemic). An explanation of the alert levels can be found at the WHO Web page on pandemic alerts.

Vaccines are now available for this strain; although the H1 and N1 antigens are similar to those in recent strains of human flu, they are not similar enough that the current flu vaccine is expected to protect people from this swine flu strain. A report from CDC suggests that people in their 50's and 60's have some immunity to this strain, and that the "new" strain is actually a descendant of the 1918 "Spanish" H1N1 flu virus, rather than the H1N1 strains of recent years. The strain appears so far to be resistant to the uncoating antivirals (amantidine and rimantadine), but sensitive to the neuramidase inhibitors (zanamivir and oseltamivir), although there have been recent reports from several countries around the world of people infected with a virus of this strain that is resistant to oseltamivir (but not zanamivir).

People in the United States who have died of this strain have generally had other medical problems, such as morbid obesity (body mass index of 35 or higher), asthma, diabetes, heart disease, or pregnancy, which made them more susceptible to complications of the flu. My personal speculations on why (and remember that I am not a microbiologist, an infectious disease specialist, or an epidemiologist):

(As I said -- these are my personal speculations...)

Preventing Swine Flu

The best preventative measures are still good hygiene. The CDC recommends:

A surprising number of people are refusing to get the H1N1 vaccine, for themselves or for their children. It's not clear why they would want not to receive the vaccine -- especially for their children, in view of the number of children who have died of H1N1 flu and its complications.

One common "problem" that I have talked about with many parents is that the H1N1 vaccine is "new". It isn't new. The H1N1 vaccine is made in exactly the same way as the seasonal flu vaccine (by growing the virus on chicken eggs); the only difference between the H1N1 and seasonal flu vaccines is in the flu virus antigens contained in each vaccine (the H1N1 vaccine contains only the H1N1 antigen, while the seasonal vaccine contains antigens to all three of this year's most likely flu strains.

Another issue is that some people remember the 1976 swine flu problem, and especially the reactions many people had to the 1976 swine flu vaccine. The vaccine people are much better at making vaccines now than they were 33 years ago, and because of that the vaccines we now have available are much safer, with fewer side effects, than they were in the '70s. I haven't seen anyone have problems with the H1N1 vaccine. (We have seen a number of children die from H1N1 infections and its complications, though.) It still appears to be much safer to receive the vaccine than to risk getting H1N1 flu.

For those people who eat pork products: it is impossible to catch swine flu by eating well-cooked pork products. Pork and pork products (such as ham and bacon) should always be cooked thoroughly anyway, to eliminate any possibility of infection with trichinosis, a parasite which is often found in pork products.

Recall of Some Pediatric H1N1 Vaccine

Two manufacturers of H1N1 vaccine have now recalled certain lots of their H1N1 vaccines after finding that the vaccines in these lots contain lower level of the H1N1 antigen (the protein in the virus that a vaccinated or exposed person must develop antibodies to in order to become immune to the virus) than there should be. Sanofi Pasteur, one of the companies manufacturing H1N1 vaccine, has recalled four lots of its injectable flu vaccine made without preservatives for use in small children, and MedImmune has recalled 13 lots of its FluMist® nasal spray vaccine.

This does NOT mean that the vaccine from those lots didn't work -- both the CDC and the Food and Drug Administration agree that the antigen level in those lots is still high enough that children who receive the vaccine will still be protected against the H1N1 strain. In fact, the recalled FluMist® vaccine was at full potency when shipped, and many, if not most, patients who received FluMist received it when it was still fully potent.

Also, there have been NO concerns about the safety of vaccine from these lots -- the low antigen concentration is the only problem with these lots of vaccine Vaccines from these lots are being recalled, but children who have had vaccine from these lots do not need to repeat the dose (although children younger than 10 years old who have had only one dose of H1N1 vaccine still need their second doses).

For more information on swine flu, see the CDC Web site section on swine flu or the WHO Web page on swine flu. For more information on flu in general, including symptoms, signs, and possible complications, see my main influenza page.

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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 Vinay N. Reddy, M.D. All rights reserved.
Written 04/29/09, based on information first posted 09/02/96 and revised multiple times between 1996 and 2009; last revised 01/04/2010 counter