How H1N1 influenza is similar to the influenza A virus

by Todd Neale

The pandemic H1N1 influenza virus is similar to seasonal influenza A viruses in terms of viral shedding, symptoms, and household transmission, researchers in Hong Kong found.

The researchers prospectively followed 99 patients testing positive in rapid diagnostic tests for influenza A virus and found 45 had the H1N1 pandemic virus and 54 were infected with a seasonal influenza A virus, H3N2, but the two strains did not differ significantly, Benjamin Cowling, PhD, of the University of Hong Kong School of Public Health, and colleagues reported in the June 10 issue of the New England Journal of Medicine.

The study also confirmed findings by numerous previous studies that younger individuals were disproportionately affected by the H1N1 pandemic virus.

But, Cowling and his colleagues wrote, “in spite of differences in the age groups affected and occasional complications in patients with underlying disease, most of the epidemiologic data from affected countries suggest that pandemic 2009 H1N1 virus infection is not, on average, a more severe illness than seasonal influenza.”

In addition to the 99 index patients, researchers collected nasal and throat swabs from 284 of their household contacts during three home visits over the course of a week, testing the samples using reverse-transcriptase polymerase chain reaction (RT-PCR).

All individuals were instructed to use a hand-hygiene intervention.

Secondary attack rates — as confirmed with RT-PCR — were similar among household contacts for the pandemic virus (8%) and the seasonal influenza A viruses (9%).

The time between the onset of illness in the index patient and the beginning of symptoms in a household contact was an average of 3.2 days for the pandemic virus and 3.4 days for seasonal H3N2.

For both the pandemic and seasonal strains, viral shedding stopped after about five to seven days of illness, and clinical symptoms generally ceased after 10 days.

A subgroup of 47 patients provided blood samples for antibody testing. Among these patients, 36% of household contacts who had serologic evidence of infection with the H1N1 pandemic virus did not shed detectable virus or display any symptoms.

Patients with confirmed infection with the pandemic virus who were treated with oseltamivir had reduced antibody titers compared with those who did not receive antiviral treatment, suggesting an increased susceptibility to reinfection.

The researchers cautioned, however, that “further studies are warranted to confirm or refute this potential association and to investigate the degree to which patients who are infected with 2009 H1N1 virus and are treated with antiviral agents are protected against reinfection in subsequent pandemics.”

A second study in NEJM by Vernon Lee, MBBS, MPH, of the Singapore Ministry of Defense and the National University of Singapore, and colleagues examined a strategy for reducing transmission of the pandemic virus in the semiclosed setting of military camps in Singapore.

The strategy involved oseltamivir chemoprophylaxis of contacts of infected individuals, close follow-up, and isolation of those with confirmed disease.

Mathematical models had predicted that so-called “ring chemoprophylaxis” would be effective, but it had not been assessed in the real world.

During four days in June 2009, before H1N1 vaccine was available, four outbreaks of the pandemic virus occurred at four Singaporean military camps, including the camp medical center.

The camps were considered semiclosed environments because the personnel were allowed to return home on weekends.

When the outbreaks began, all personnel with suspected infection were tested and, if infection was confirmed, patients were isolated and treated with oseltamivir.

All members of the same military unit, regardless of whether they had had close contact with the infected patient, were given oseltamivir prophylaxis for 10 days.

All members of the affected units were screened three times weekly for subclinical infections using nasopharyngeal swabs and RT-PCR.

The four outbreaks involved 1,175 personnel, and 1,100 received oseltamivir prophylaxis.

After the intervention was introduced, the rate of infection dropped from 6.4% of camp personnel to 0.6% (P<0.001) overall. Three of the four sites saw significant reductions after the ring chemoprophylaxis intervention was implemented.

There was also a significant reduction in the overall reproductive number, from 1.91 to 0.11 (P<0.001), when confirmed cases were considered.

No serious adverse events were reported with oseltamivir, although 7.7% of those treated reported mild, nonrespiratory side effects.

Although vaccine is the first-line option for preventing influenza transmission, Lee and his colleagues wrote, “antiviral prophylaxis may be considered as an additional strategy in reducing the pandemic’s effects, especially in areas in which the supply of vaccine is limited. Furthermore, this strategy may be important in future epidemics and pandemics, either before vaccines are available or when there is a poor match between the vaccine and circulating strains.”

In an accompanying editorial, Timothy Uyeki, MD, MPH, MPP, of the CDC’s Influenza Division in Atlanta, noted that the findings of the Singapore study “do not directly inform the success of potential containment efforts implemented at the source of the next influenza pandemic or implemented to prevent the introduction of influenza into a community.”

Referring to the findings from both Hong Kong and Singapore, he concluded, “Since transmission appears to be similar for 2009 H1N1 and seasonal influenza A viruses, these studies and others suggest that interventions that are recommended for the control of seasonal influenza outbreaks in institutions are also likely to be effective for the control of outbreaks of 2009 H1N1 virus infection but that the maximum benefit is achieved when outbreaks are identified early and multiple measures are implemented simultaneously, as soon as possible.”

Todd Neale is a MedPage Today staff writer.

Originally published in MedPage Today. Visit for more flu news.