Study explores Southern Hemisphere pandemic patterns

Oct 5, 2010 (CIDRAP News) – A review of how the first wave of the 2009 H1N1 pandemic affected Southern Hemisphere countries found many similarities with Northern Hemisphere countries, though many patients had no underlying risk factors for flu complications and pregnant women didn't seem to have severe outcomes.

The authors from Greece and from Tufts University School of Medicine in Boston based their findings on 15 studies in the medical literature that described the epidemiologic findings in the Southern Hemisphere. They published their findings today in an early online posting by Epidemiology and Infection.

Seven of the studies detailed the experience of South American countries, six covered Australia and New Zealand, and two focused on Africa.

The Southern Hemisphere experienced its first pandemic wave during the region's 2009 winter influenza season (northern summer). Some countries, such as New Zealand and Australia, recently saw or are in the midst of a second pandemic wave. Australia's health ministry said in its most recent report that flu activity is increasing, with nearly 70% of cases caused by the 2009 H1N1 virus. The rise in cases there is unusual, coming at the end of the flu season.

All but one of the studies included in the literature review contained data on the characteristics of patients with lab-confirmed 2009 H1N1 infections.

In patients whose respiratory samples were evaluated with reverse -transcript polymerase chain reaction (RT-PCR) testing, the rate of positives ranged from 31.5% to 54% in four studies from Brazil, Argentina, and Australia, though a reference laboratory in Bolivia reported a lower rate of 12.7%. The positive rate was even lower, about 8.3%, in two Brazilian studies, and a general hospital in Argentina has a 3.3% positive rate.

Hospitalization rates in confirmed cases were about 45% in Brazil and Argentina and varied from about 17% to 31% in Australia and New Zealand.

Among five studies that included data on fatality levels, rates ranged from 0.5% to 0.9%.

The majority of lab-confirmed 2009 H1N1 cases were in young and middle-aged adults, with the second highest levels of infections in older children and adolescents. Though fewer seniors were affected, the ones who were sickened were more likely to be hospitalized, to be admitted to an intensive care unit, or to die. .

Nearly 67% of patients with severe infections did not have underlying medical conditions. Common conditions in patients that did have risk factors for flu complications included, for example, chronic respiratory disease such as asthma, cardiovascular disease, renal insufficiency, and diabetes.

Studies that included information on pregnant women reported that this group accounted for between 5.4% and 8.1% of severe infections. However, they didn't find that pregnant women were substantially more likely to be hospitalized or admitted to the ICU. "In other words, pregnancy might be mostly a risk factor for acquisition of infection with pandemic A (H1N1) 2009 influenza virus, rather than for an adverse outcome, " the researchers wrote.

They noted that pregnant women were more likely to be in the age-group most affected by the virus and may often have close contact with young children, who could expose them to the virus.

The researchers found that a substantial number of obese people were sickened during the pandemic wave, a pattern that also emerged in the United States and other Northern Hemisphere countries. However, the group emphasized that their findings concerning pregnant and obese patients were just observations.

They also pointed out that most cases of pandemic flu were mild and uncomplicated and went undiagnosed. Because their review focused on lab-confirmed cases, they said, it doesn't capture the true impact and characteristics of the 2009 flu pandemic in the Southern Hemisphere.

See also:

Falagas ME, Koletsi PK, Baskouta E, et al. Pandemic A(H1N1) 2009 influenza: a review of the Southern Hemisphere experience. Epidemiol Infect 2010 Oct 5 (early online publication) [abstract]

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