National Influenza Surveillance in the Philippines from 2006 to 2012: seasonality and circulating strains.
BACKGROUND
The results of routine influenza surveillance in 13 regions in the Philippines from 2006 to 2012 are presented, describing the annual seasonal epidemics of confirmed influenza virus infection, seasonal and alert thresholds, epidemic curve, and circulating influenza strains.


METHODS
Retrospective analysis of Philippine influenza surveillance data from 2006 to 2012 was conducted to determine seasonality with the use of weekly influenza positivity rates and calculating epidemic curves and seasonal and alert thresholds using the World Health Organization (WHO) global epidemiological surveillance standards for influenza.


RESULTS
Increased weekly influenza positive rates were observed from June to November, coinciding with the rainy season and school opening. Two or more peaks of influenza activity were observed with different dominant influenza types associated with each peak. A-H1N1, A-H3N2, and two types of B viruses circulated during the influenza season in varying proportions every year. Increased influenza activity for 2012 occurred 8 weeks late in week 29, rather than the expected week of rise of cases in week 21 as depicted in the established average epidemic curve and seasonal threshold. The intensity was severe going above the alert threshold but of short duration. Southern Hemisphere vaccine strains matched circulating influenza virus for more surveillance years than Northern Hemisphere vaccine strains.


CONCLUSIONS
Influenza seasonality in the Philippines is from June to November. The ideal time to administer Southern Hemisphere influenza vaccine should be from April to May. With two lineages of influenza B circulating annually, quadrivalent vaccine might have more impact on influenza control than trivalent vaccine. Establishment of thresholds and average epidemic curve provide a tool for policy-makers to assess the intensity or severity of the current influenza epidemic even early in its course, to help plan more precisely resources necessary to control the outbreak. Influenza surveillance activities should be continued in the Philippines and funding for such activities should already be incorporated into the Philippine health budget.
['Lucero|Marilla G|MG|', 'Inobaya|Marianette T|MT|', 'Nillos|Leilani T|LT|', 'Tan|Alvin G|AG|', 'Arguelles|Vina Lea F|VL|', 'Dureza|Christine Joy C|CJ|', 'Mercado|Edelwisa S|ES|', 'Bautista|Analisa N|AN|', 'Tallo|Veronica L|VL|', 'Barrientos|Agnes V|AV|', 'Rodriguez|Tomas|T|', 'Olveda|Remigio M|RM|']
D000293:Adolescent D000328:Adult D002648:Child D002675:Child, Preschool D004196:Disease Outbreaks D058872:Epidemics D006801:Humans D007223:Infant D053118:Influenza A Virus, H1N1 Subtype D053122:Influenza A Virus, H3N2 Subtype D007252:Influenza Vaccines D007251:Influenza, Human D008875:Middle Aged D010679:Philippines D011159:Population Surveillance D012189:Retrospective Studies D012574:Schools D012621:Seasons D013997:Time Factors D055815:Young Adult
Alert threshold Circulating influenza strains Influenza seasonality Average epidemic curve Seasonal threshold Influenza vaccine strains