Influenza surveillance and immunisation in New Zealand, 1997-2006.

BACKGROUND: The national influenza surveillance in New Zealand is an essential public health component for assessing and implementing strategies to control influenza. OBJECTIVE: The aim of this study is to report the national influenza surveillance data collected during 1997-2006 in terms of the community disease burden, circulating viral strains, hospitalisations, mortality, and immunisation coverage. METHODS: The national influenza surveillance system includes sentinel general practice surveillance, laboratory-based surveillance, and hospital admission and mortality surveillance and immunisation coverage. The results obtained during 1997-2006 were analysed. RESULTS: When the last 10 years were compared to the previous years, sentinel general practice surveillance recorded a decreasing trend of influenza-like illness rates in the community. Sentinel surveillance also showed that children aged 0-4 years were the most affected. Influenza-related hospitalisation surveillance reported an increasing trend of hospital admissions particularly in children aged 0-19 years. Introduction of routine influenza vaccination among the New Zealand elderly was associated with a significant decrease of influenza-related mortality. CONCLUSIONS: This report demonstrates that an integrated virological and epidemiological surveillance system for influenza is essential for monitoring the disease burden, identifying circulating strains, guiding effective vaccination and planning for a potential pandemic.

Influenza surveillance in New Zealand in 2005.

AIM: We report the influenza activity in New Zealand in 2005 (including an influenza B epidemic) in terms of the disease burden, hospitalisations, viral strain characterisations, and vaccine recommendations. METHODS: The national influenza surveillance system includes sentinel general practice surveillance, laboratory-based surveillance, and hospital admission and mortality surveillance. The results obtained in 2005 were analysed. RESULTS: During the 2005 winter season, 3929 consultations for influenza-like illness (ILI) were reported from a national sentinel network of 87 general practices. It is estimated that ILI resulting in a visit to a general practitioner affected over 47,108 New Zealanders or about 1.3% of total population. Influenza hospitalisations and viral isolations reached the second highest level in the 15 years from 1990 to 2005. Influenza morbidity (as measured by age specific rates of hospitalisations) was high in children aged 0-19 years. In particular, the burden of influenza in children aged 5-19 years in 2005 was higher than previous years from 1995-2004 as measured by the excess morbidity rate and viral isolations. The ILI consultation rates varied greatly among health districts with the highest rates being reported from the Eastern Bay of Plenty and Otago Health Districts. The influenza activity peaked in the mid of June to July with influenza B activity preceding influenza A activity. This influenza B activity was the highest level recorded over the last 15 years. Influenza B/HongKong/330/2001-like virus was the predominant strain. Significant antigenic drift was observed among the A/Wellington/1/2004 (H3N2)-like viruses and B/HongKong/330/2001-like viruses--resulting in an updated seasonal vaccine strain for 2006. CONCLUSION: The influenza surveillance in 2005 recorded the highest influenza B activity over the last 15 years with co-circulation of influenza B (Hong Kong) and B (Shanghai) strains in an epidemic. The peak of influenza B activity preceded the peak of influenza A activity with significant antigenic drift among the A/Wellington/1/2004 (H3N2)-like viruses and B/HongKong/330/2001-like viruses. Significant excess morbidity was observed in the 5 to 19 year age group in a highly variable geographical distribution across New Zealand. This confirms the value of the national influenza surveillance system as an essential public health component for monitoring the incidence and distribution of influenza and predominant strains in New Zealand.