Medical Grand Rounds

Update on 2009 pandemic influenza A (H1N1) virus

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ABSTRACTThe pandemic of a novel strain of swine-origin influenza A (H1N1) is expected to make this a difficult flu season. Fortunately, this strain is relatively mild, and the principles of prevention, diagnosis, and treatment remain the same. Physicians will have a number of complex decisions to make about when to test, when to treat, and when to simply reassure.


  • Vaccination this season will require two vaccines: a trivalent vaccine for seasonal influenza and a monovalent vaccine for 2009 pandemic influenza A (H1N1).
  • Recent studies indicate that the monovalent vaccine for 2009 pandemic influenza A (H1N1) may require only one injection.
  • To date, 2009 pandemic influenza A (H1N1) virus has not been exceptionally virulent and differs from conventional influenza in that it seems to disproportionately affect children and young adults. Pregnant women are at a higher risk of complications.
  • Most people with 2009 pandemic influenza A (H1N1) do not need to be tested, treated, or seen by a clinician.
  • Antiviral drugs should be reserved only for those at high risk of influenza complications.



A 69-year-old ohio man with leukemia was treated in another state in late June. During the car trip back to Ohio, he developed a sore throat, fever, cough, and nasal congestion. He was admitted to Cleveland Clinic with a presumed diagnosis of neutropenic fever; his absolute neutrophil count was 0.4 × 109/L (reference range 1.8–7.7). His chest radiograph was normal. He was treated with empiric broad-spectrum antimicrobials. On his second day in the hospital, he was tested for influenza by a polymerase chain reaction (PCR) test, which was positive for influenza A. He was moved to a private room and started on oseltamivir (Tamiflu) and rimantadine (Flumadine). The patient’s previous roommate subsequently tested positive for influenza A, as did two health care workers working on the ward. All patients on the floor received prophylactic oseltamivir.

The patient’s condition worsened, and he subsequently went into respiratory distress with diffuse pulmonary infiltrates. He was transferred to the intensive care unit, where he was intubated. Influenza A was isolated from a bronchoscopic specimen. He subsequently recovered after a prolonged course and was discharged on hospital day 50. Testing by the Ohio Department of Health confirmed that this was the 2009 pandemic influenza A (H1N1) virus.


We are now in the midst of an influenza pandemic of the 2009 influenza A (H1N1) virus, with pandemic defined as “worldwide sustained community transmission.” The circulation of seasonal and 2009 pandemic influenza A (H1N1) strains will make this flu season both interesting and challenging.

The approaches to vaccination, prophylaxis, and treatment will be more complex. As of this writing (mid-September 2009), it is clear that we will be giving two influenza vaccines this season: a trivalent vaccine for seasonal influenza, and a monovalent vaccine for pandemic H1N1. It appears the monovalent vaccine may require only one dose to provide protective immunity.1 Fortunately, the vast majority of cases of pandemic H1N1 are relatively mild and uncomplicated. Still, some people are at higher risk of complications, including young patients, pregnant women, and people with immune deficiency or concomitant health conditions that put them at higher risk of flu-associated complications. Thus, clinicians will need to be educated about whom to test, who needs prophylaxis, and who should not be treated.

As our case demonstrates, unsuspected cases of influenza in hospitalized patients or health care workers working with influenza pose the greatest threat for transmission of influenza within the hospital. Adults hospitalized with influenza tend to present late (more than 48 hours after the onset of symptoms) and tend to have prolonged illness.2 Ambulatory adults shed virus for 3 to 6 days; virus shedding is more prolonged for hospitalized patients. Antiviral agents started within 4 days of illness enhance viral clearance and are associated with a shorter stay.3 Therefore, we should have a low threshold for testing for influenza and for isolating all suspected cases.

This is also creating a paradigm shift for health care workers, who are notorious for working through an illness. If you are sick, stay home! This applies whether you have pandemic H1N1 or something else.


The location of cases can now be found on Google Maps; the US Centers for Disease Control and Prevention (CDC) provides weekly influenza reports at

Pandemic H1N1 appeared in the spring of 2009, and cases continued to mount all summer in the United States (when influenza is normally absent) and around the world. In Mexico in March and April 2009, 2,155 cases of pneumonia, 821 hospitalizations, and 100 deaths were reported.4

In contrast with seasonal influenza, children and younger adults were hit the hardest in Mexico. The age group 5 through 59 years accounted for 87% of the deaths (usually, they account for about 17%) and 71% of the cases of severe pneumonia (usually, they account for 32%). These observations may be explained in part by the possibility that people who were alive during the 1957 pandemic (which was an H1N1 strain) have some immunity to the new virus. However, the case-fatality rate was highest in people age 65 and older.4

As of July 2009, there were more than 43,000 confirmed cases of pandemic H1N1 in the United States, and actual cases probably exceed 1 million, with more than 400 deaths. An underlying risk factor was identified in more than half of the fatal cases.5 Ten percent of the women who died were pregnant.

Pandemic H1N1 has several distinctive epidemiologic features:

  • The distribution of cases is similar across multiple geographic areas.
  • The distribution of cases by age group is markedly different than that of seasonal influenza, with more cases in school children and fewer cases in older adults.
  • Fewer cases have been reported in older adults, but this group has the highest case-fatality rate.


Next Article:

Influenza in long-term care facilities: Preventable, detectable, treatable

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