Influenza A/Mexico/2009 (H1N1): Questions and answers

questionHere are my answers to questions about the currently circulating influenza H1N1 strain (formerly swine flu) sent by readers of virology blog.

A reader from Mexico shared the following numbers with virology blog, then asked a question about hog cholera.

Q: There are 312 confirmed cases of swine virus here in Mexico and 12 of them have died. This means 3.8%. In the rest of the world there are 159 confirmed cases of swine virus infection, but no deaths. It gets even worse, there have been reported 176 deaths by pneumonia in this past two weeks, but only 12 are positive for the swine virus, this means that only 6.8% of the fatalities can be blamed on the swine virus. Three states San Luis Potosi, Oaxaca and Aguascalientes have together reported 195 cases of pneumonia with 24 deaths, that is 12.3% die. While three other states (Tlaxcala, Veracruz and Coahuila) have together reported 154 cases of pneumonia with no deaths…(a similar situation as the rest of the world).

I have read that hog cholera can increase the death toll of influenza virus above 10%. What is hog cholera and why does this worsen flu?

A: Hog cholera is an infectious disease of pigs caused by a virus known as classical swine fever virus. Infection with this virus leads to fever, skin lesions, convulsions and often death. It is also immunosuppressive, therefore worsening influenza in swine.

Q: How risky is it to travel internationally (to places other than Mexico) at this time? How likely is one to pick up a flu on an airplane, in an airport, while staying at a hotel or hostel, etc? I am supposed to be visiting Spain for a month – my travel insurance company says they won’t cover any cancellation costs because no one has issued a travel advisory against Spain. Does this mean that it is more or less safe to visit Spain, although there are cases reported there as well? I am confused. How likely is one to become sick while travelling? Would it be better to stay home at this point?

A:At this point the new influenza virus strain seems to be nearly everywhere, even in the US, based on suspected cases. Therefore travel poses no increased risk. However, travel is a good way to become infected – airports, hotels, airplane, are all good venues for transmission by aerosol or contact (see an earlier virology blog post on transmission – the airplane transmission story is particularly interesting). So in my view, you probably have a higher risk of being infected because you will be in contact with more people while traveling than if you stayed at home. But if you take precautions like cleaning hands with an alcohol-based cleaner, refrain from touching mouth/eyes/nose, you can minimize the likelihood of exposure. I doubt there will be travel advisories issued for Spain because by the time there are many cases there, the virus will be elsewhere as well. If your trip is not imminent, you can delay and make a decision later.

Q: Is Vitamin D effective against influenza? And does this also help explain seasonality?

A: It has been hypothesized that vitamin D was effective against influenza (see this paper), but this has never been rigorously proven. Seasonality better correlates with temperature and humidity.

Q: Your note that CDC does not recommend use of face masks for reducing viral spread is somewhat inaccurate. In addition, the posted article titled “Influenza Viral Transmission” does not take transmission through contact with GI substances into account. In CDC’s Interim Recommendations for Facemask and Respirator Use in Certain Community Settings Where Swine Influenza A (H!N!) Virus Transmission has been detected, it is stated, “When crowded settings and close contact with others cannot be avoided, the use of facemasks or respirators in areas where transmission of swine influenza A (H1N1) virus has been confirmed should be considered. The documents then gives three circumstances.

A: Thank you for your comments and support. I understand that in some cases facemasks/respirators are recommended. As I pointed out in virology blog, on the front page of CDC, which most laypersons will see, there is no recommendation for the use of such protection. As you know, when not use properly, face masks do not function as intended and most laypersons will not have the opportunity to use respirators (by layperson I mean someone not involved in health care). As for GI spread, influenza cannot and does not replicate in the GI tract of humans (although it does so in birds). Concurrent infections with other pathogens (e.g. norovirus) may lead to GI symptoms.

Q: Our elite team of news reporters (who still refer to this as Swine flu) listed vomiting and diarrhea as symptoms. This seems peculiar for flu. During the regular flu season, I remember telling patients to look out for fever, arthralgias, cough etc…never anything about vomiting and diarrhea. Is this a legitimate symptom, or just the media being as vague as possible?

A: Influenza cannot and does not replicate in the GI tract of humans (although it does so in birds). Concurrent infections with other pathogens (e.g. norovirus) may lead to GI symptoms.


A: Do not attempt to become infected now; the outcome of influenza infection may not be benign.

Q: I’d like to know the potential lethality of swine influenza  as compared with other types of influenza viruses. IS THERE ANY REFERENCE ABOUT THE RISK OF INFECTION IN OPEN OR CLOSED SPACES??

A: There is no reason to believe that the current H1N1 virus, which originated in swine, is any more lethal than any other influenza virus, with the exception of avian H5N1 strains and the 1918-19 strain. Transmission is likely to be more efficient in closed spaces (such as an airline cabin; see this post).

Q: How long would you expect human immunity to exist within an individual infected with H1N1 virus?

A: I would expect immunity to be lifelong against that specific strain. However, immunity will not completely protect agains future antigenically drifted viruses of the same lineage.

Q: One small correction. N-95s are respirators, not face masks. Respirators must meet specifications for filtering out specific types and sizes of particles. An N-95 is a particulate respirator that when fitted properly filters 95% of particles 0.3 microns or larger. Face masks generally refer to medical and surgical masks that need only provide a physical barrier.

Thank you.

Q: At the end of a recent interview with Science magazine, Dr. Donis of the CDC states that the USA has received 300 samples of this virus from Mexico. Why has the CDC failed to publish the sequences of any of these 300 Mexican samples Dr. Donis says have been received from Mexico? Doesn’t this seem to be an intentional act?

A: I’m not sure. It’s been suggested that the Mexican government wants to keep rights to the use of the viruses as a vaccine and therefore do not want the sequence released. I suspect we’ll find out in the coming week.

Q: Regarding Dr. Schuchat’s statement, could it be that if you trace the lineage of certain viral genome segments that their origin could be avian, for example? i.e., perhaps the A/swine/Indiana/P12439/00 isolate was/is a reassortant originally containing some segments of avian origin? Just a thought, I have not personally looked closely at these sequences…

and also:

Q: But you still agree that this virus appears to contain genetic pieces from from four virus sources, correct? (one avian influenza viruses, one human influenza viruses, and two swine influenza viruses).

A: In 1998, recombinant viruses arose in pigs that were a combination of human, pig, and avian influenza virus RNAs. See Dr. Donis’ statement in yesterday’s Science interview. Today they look like swine viruses because they have been evolving in this animal for so many years.

Q: How do we account for the April 24 comments of Dr. Marie Gramer below, who has perhaps the largest library of swine flu virus isolates in the nation the day after Schuchat’s comments? “There have been no reports of this virus in pigs, said Dr. Marie Gramer, a swine flu expert with the University of Minnesota’s college of veterinary medicine. “It doesn’t seem to be very similar to anything that is currently circulating, from what I have,” said Gramer, who has an extensive library of swine flu virus isolates.”

A: I believe she is wrong.

Q: I do not understand why you are morally certain that the German and New York isolates arose from Mexico.

A: I am not certain, I only assume that because the New York and German tourists visited Mexico, that there were infected there and returned with the virus. However the point is moot if the Mexican viruses are very similar to all the others.

Q: Mexico’s chief epidemiologist insists this virus did not begin in a Mexican pig farm. “Miguel Angel Lezana, Mexico’s chief epidemiologist, told reporters…. the presence of Eurasian swine flu genes in the H1N1 virus makes it unlikely that the disease originated in a Mexican pig farm.”

A: Chief Epidemiologist? He should be fired. The fact that Eurasian swine flu genes are present in no way makes a Mexican origin more unlikely than any other.

Q: It’s taken me a while to digest the fact that the Mexican flu viruses haven’t been sequences. Shouldn’t this be bigger news? Why is it confined to a relatively obscure blog? (No offense, Dr. Racaniello.)

(I could not resist printing the preceding question. Not offended at all!)

Q: Canine influenza emerged a few years ago in Florida in greyhounds, and was found to have come from an equine influenza strain.  However, after entering the canine population, the virus is no longer able to infect horses.  Perhaps a similar thing is occurring here.

A: Swine are readily infected with human influenza viruses. So there is no reason to believe that the new human H1N1 virus would not be able to go back into pigs. We’ll see.

Q: Fucoidan is a sulfated polysaccharide primarily found in brown seaweeds.  Although the specific shape/sugar to sulfate compositions vary from seaweed to seaweed, the fucoidan used in this paper is from Undaria pinnatifida, one of the most popular dietary seaweeds consumed in Japan and Korea.  The sporophyll contains 8-12% fucoidan (the part of the Undaria used in this study, also called Mekabu in Japanese).  Is it possible that people in Japan will be protected from H1N1 if they eat Mekabu?

A: The concentrations of fucoidan used in the studies you provided are rather high and would not be present at such levels in Mekabu. Therefore eating the seaweed would not likely be of value in preventing infection.

Comments on this entry are closed.

  • Matt Dubuque 1 May 2009, 8:13 pm

    I agree. The chief epidemiologist of Mexico should be fired!

  • carlos guevara-casas 1 May 2009, 9:17 pm


  • ppga 2 May 2009, 1:26 am

    Hello Professor –

    Great blog, thank you for this wealth of information. My question in this – I understand that with this new, novel strain of virus most, if not all people will not have immunity from previous exposure to the established strains. However, many (or maybe just some people) will either avoid infection, or become infected yet experience no remarkable symptoms, or maybe just exhibit very mild symptoms rather than get really seriously ill. What is at work when a person actually is infected yet not really “sick”? A good immune system that allows infection but contains it quickly? or something else at work? Or just luck at avoiding virus particles?

    Example – I am 36 years old, and have never been diagnosed, or as far as I know, contracted influenza. I have had many other common ailments (many bouts of strep and colds, chicken pox, mononucleosis) over my years, but seemingly not flu virus. I have never been vaccinated against flu. What could account for this? Luck? I find it impossible I haven't ever come in contact with flu virus. Assuming I have had contact with flu virus, why no remarkable illness for me? Clearly other infections can occur in me. Are some people “immune” to all types of flu or react to it differently than others who suffer the common symptoms? Anyways thanks for the great website.

  • karbon 2 May 2009, 3:32 am

    Informative blog.

  • safdar 2 May 2009, 6:38 am

    your blog site is very nice so plz check my blog site and give me more comments.

  • phogdog 2 May 2009, 6:48 am

    Thanks. Another great set of Q's and A's.

  • diana 2 May 2009, 7:42 am

    I'm honored that you reprinted my question – now, can you answer it with a simple “yes” or “no”?

    “Shouldn’t this be bigger news?”

  • kookygirl 2 May 2009, 8:01 am

    I'm getting over a flu and want to test if it's swine flu. How would I go about it when almost healthy? Would a nasal swab work if there's hardly anything?

  • Robert Innes 2 May 2009, 11:45 am

    Dear Dr. Racaniello,

    Thank you for your continuing updates and FAQs. The MSM treatment of the pandemic seems ill-informed and thus full of contradictory claims, and your blog has become my go-to resource. Three issues in particular have been inadequately addressed elsewhere.

    First, what is the link, if any, between lack of immunity to a virus and the virulence or morbidity of the resulting viral disease? In other words, should we fear a pandemic because: (1) more people will come down sick than during a normal flu season due the unfamiliarity of the virus; or (2) a higher percentage of the people who get sick will die; or (3) both?

    Second, with respect to airplane travel and travel generally, I think the press and the authorities have done a very poor job distinguishing between systemic risk and personal risk. Indeed, politicians in particular are hopelessly conflicted on this subject, notwithstanding the vice president's widely denounced burst of candor. Is it the case that: (1) while international travel poses no systmetic risk of spreading the virus, which is already present in North and South America, Europe and Asia, (2) individuals nonetheless greatly increase their chances of catching viral disease by traveling on airplanes, staying in hotels, etc.?

    Finally, I have seen little intelligent discussion of the vastly different countermeasures deployed by different governments. While everyone scoffs at Egypt, for instance, for slaughtering hundreds of thousands of pigs, no one mentions the radically different responses of the U.S. and Hong Kong authorities. Has Hong Kong simply got the science wrong when they decide to quanrantine hundreds of hotel guests for 10 days?

    Best regards,

  • Hugo Montaldo 3 May 2009, 7:51 am

    Besides inherent deficiencies in the Mexican health system and initial
    delays by patients and doctors for a correct diagnosis, there is
    another scaring possibility for the high death rate in Mexico from the
    swine flu virus. That possibility is related to the instructions given
    to the doctors by the Ministery of Health to refrain to treat people
    with antivirals (Tamiflu, Relenza), unless they already have pulmonary
    signs (so too late), or they have a chronic disease in addition to the
    swine flu. Such instruction might save middle aged people (those with
    chronic diseases) and causes many neumonia cases among the more
    healthy youngsters. That may parcially explain why not only the death
    rate is so high, but also the age structure of the affected
    population. We'll see if we can solve this questions in the future,
    because they are full of political consequences.

  • profvrr 3 May 2009, 6:35 pm


  • profvrr 3 May 2009, 6:39 pm

    He should be fired because he doesn't understand that the presence of
    Eurasian flu genes in this virus has no bearing whatsoever on the
    origin. It may well be that some pigs in Mexico have viruses with such
    genes. If he doesn't understand this simple concept, he should not be
    Minister of Health.

  • gsgs 11 May 2009, 8:05 am

    when was
    A/Mexico/InDRE4487/2009(H1N1) isolated ?

    how can it be that there are only 2 mutations to the calculated index case,
    when the first viruses were confirmed in mid April in Mexico ?

  • profvrr 15 May 2009, 8:37 am

    According to NCBI InDRE4487 was isolated in April 2009. The GISAID
    entry date is probably incorrect.

  • Don 7 October 2009, 6:59 pm

    My husband has scheduled a cruise for the Mexican Rivera, does travel to Mexico increase my chances and my childrens chances of getting H1N1, relative to staying at home with our normal routine, which includes going out in public? Would your answer be different if I was pregnant? Our 2 year old was also a cardiac baby, he was born with transposition of the major arteries, does this change the answer? Thanks for your thoughts.

  • profvrr 23 October 2009, 5:22 pm

    Travel is definitely a risk factor for H1N1 infection. One infected
    person on an airplane is sufficient to infect 70% of the passengers.
    So if I were at risk (pregnancy, cardiopulmonary complications) I
    would stay home. Clearly you can still be infected at home but at
    least you can have some control over your movements and interactions
    with others.

  • Don 7 November 2009, 10:32 pm

    Thanks for your comments

  • Don 7 November 2009, 10:33 pm

    Thnaks for your comments

  • Don 8 November 2009, 6:32 am

    Thanks for your comments

  • Don 8 November 2009, 6:33 am

    Thnaks for your comments

  • Delayed Diagnosis 8 June 2011, 11:07 am

    Although your doctor or other medical practitioner may be responsible for the medical negligence you have suffered, they are not sued directly. Action is always taken against their employer – the practice, hospital or health authority for whom they work.