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Epidemic flu strains in South America later predominate in North America
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Influenza virus strains identified during epidemics in South America may frequently become predominant in later epidemics in Central and North America, according to recent findings.
Researchers accessed the Global Influenza Surveillance and Response System for data on the number of respiratory swabs tested annually and the number that tested positive for influenza between 2002 and 2013 in North American, Central American and South American countries. Percentages of respiratory specimens that tested positive for influenza each month were determined to calculate annual medians. Months in which the percentage exceeded the annual median were classified as epidemic. The researchers also evaluated whether virus strains identified as predominant in South America later were detected in Central and North America, and whether strains detected in North America later were confirmed in Central and South America.
From 2002 to 2013, South America reported 877,770 respiratory samples positive for influenza (2.8 per 10,000 persons annually), and North America reported 4,535,508 cases (9 per 10,000 persons annually). From 2006 to 2013, Central America reported 82,163 results (2.4 per 10,000 persons annually).
Influenza epidemics began in South America in April, in Central America in June, and in North America in December. Influenza A(H1N1) virus was the most prevalent influenza virus in South America, predominating in all nine seasons in Central America and 92% (95% CI, 78%-107%) of 13 later seasons in North America. Likewise, influenza A (H3N2) strains in South America were predominant in 92% (95% CI, 76%-107%) of 12 ensuing seasons in Central America and 71% (95% CI, 48%-95%) of 14 ensuing seasons in North America.
In North America, the predominant influenza A (H1N1) virus strains later predominated in 78% (95% CI, 51%-105%) of nine seasons in Central America and 83% (95% CI, 62%-104%) of 12 subsequent seasons in South America. Influenza A(H3N2) virus strains in North America predominated in 67% (95% CI, 40%-93%) of 12 ensuing seasons in Central America and 77% (95% CI, 54%-100%) of 13 ensuing seasons in South America. Influenza B virus strains in North America predominated in 75% (95% CI, 51%-100%) of 12 subsequent Central American seasons and 54% (95% CI, 27%-81%) of 13 ensuing seasons in South America.
“Health officials in North and Central America may find clues about which influenza A virus strains are likely to predominate during an upcoming season by observing which were predominant in South America, and vice versa,” the researchers wrote. “Our findings underscore the need to share timely and representative specimens with WHO Collaborating Centers.” – by Jen Byrne
Disclosure: The researchers report no relevant financial disclosures.
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