Skip Navigation

Update on Swine Influenza 

 By The Staff of the Center for Biosecurity, April 24, 2009, 6:30 PM EST

After publication of this morning’s CBN Report (9:36 AM), various news articles citing officials from the World Health Organization (WHO)1 reported approximately 60 fatalities and hundreds of cases of suspected swine influenza A in humans in Mexico. 

According to the WHO, the Government of Mexico reports that the number of cases of influenza-like illness (ILI), which they have been detecting since March 18, has increased steadily. At this time, Mexico is reporting 3 separate outbreaks of ILI, which includes pneumonia:

  • In Mexico City, 854 cases of pneumonia have been reported, of those, 59 have died.

  • In San Luis Potosi in Central Mexico, 24 cases of ILI and 3 deaths have been reported.

  • In Mexicali, near the border of the U.S., 4 cases of ILI and no deaths have been reported.

What follows is a summary of what has been reported as of this writing (6:30 PM). The WHO also reports that 18 of the Mexican cases have been laboratory confirmed in Canada as swine influenza A/H1N1. Of those, 12 are genetically identical to the swine influenza A/H1N1 viruses obtained from cases in the U.S. A majority of the cases in Mexico have occurred in otherwise healthy young adults. At this point, there has been no official confirmation of the total number of ILI cases or the number of deaths reported to date that are attributable to the swine influenza virus.

In a conference call this afternoon, the CDC confirmed that swine influenza A/H1N1 is present in the specimens from the case patients in Mexico. CDC Acting Director, Richard Besser, reported that the CDC has tested 14 samples from Mexico, of which 7 match the viruses found in Texas and California. It is unclear whether these 7 samples are included among the 18 laboratory-confirmed cases of swine influenza cited by the WHO. 

The CDC also reported that ongoing surveillance has identified an additional case of swine influenza in the U. S., raising the total number of U.S. cases to 8. CDC believes this is an underestimate of the true disease burden.

In conference calls over the past 2 days, CDC has also confirmed the following: 

  • U.S. illnesses occurred between the end of March and mid-April 2009 in Southern California (Imperial and San Diego counties) and Texas (San Antonio).

  • U.S. patients range in age from 7 to 54 years.

  • The sole patient hospitalized in the U.S. was an immunosuppressed adult woman suffering from autoimmune hepatitis.

  • All 8 U.S. patients have recovered.

  • Antiviral treatment was provided to 1 U.S. patient.

  • No exposure to swine or poultry has been identified in any of the 8 U.S. cases.

  • The incubation period is estimated to be 1 to 7 days.

  • Epidemiological linkages have been established for 2 pairs of patients: 2 cases in California occurred in a father/daughter pair, and 2 cases in Texas occurred in 16 year old boys who attended the same school.

  • Nausea and diarrhea are common clinical features in the U.S. cases.

  • Only 1 of the 8 U.S. patients was vaccinated for seasonal influenza, and preliminary immunologic studies have not shown substantial protection with the current seasonal influenza vaccine.

  • As with any novel influenza virus, the CDC is creating from the U.S. swine influenza virus a seed strain that may be considered a potential candidate for an influenza vaccine.

  • Disease appears to be more severe in cases occurring in Mexico. The cause of the apparent disparity in the severity of illness is not known.

  • At this time, the U.S. is not issuing travel restrictions. The CDC will issue a travelers outbreak alert that will contain basic precautionary guidance (i.e., hand washing, respiratory etiquette). 

  • The WHO has not made any change to the pandemic alert status, which continues to be at stage 3.

The CDC has also issued Interim Guidance on Infection Control and Antiviral Recommendations for Patients with Confirmed or Suspected Swine Influenza A Virus Infection.

It is a matter of high priority to answer a number of questions, including:

  • What is the size and spread of the outbreak in Mexico and the U.S.?

  • What portion of the severe influenza-like illness occurring in Mexico is seasonal influenza? What portion is swine influenza?

  • What are the demographics of the case patients in the U.S. and Mexico?

  • What is the epidemiology and natural history of cases in the U.S. and Mexico? In particular, are there links between patients?

  • Are the confirmed swine influenza cases in Mexico more severe than in the U.S.? If so, why?

  • Over what period of time have swine influenza infections been occurring in Mexico and in the U.S.?

  • Are all or most of the cases of swine influenza closely related genetically?

  • Does the current H1N1 vaccine provide partial immunity?

1World Health Organization. Influenza-Like Illness in the United States and Mexico. April 24, 2009. Accessed April 24, 2009.