The Influenza Surveillance Program in Western Australia, 2003

Area of Science:

• Epidemiology
• Virology
• Public Health

Background:

• Western Australia experienced its most substantial influenza epidemic in at least five years during the winter of 2003.
• Influenza activity peaked sharply in August and September, characterized by a short season with high case numbers during peak weeks.

Purpose of the Study:

• To analyze the characteristics of the 2003 Western Australian influenza epidemic.
• To assess the effectiveness of the influenza vaccine and identify other respiratory viruses causing influenza-like illness.

Main Methods:

• Surveillance of influenza cases through sentinel general practices and routine laboratory detections.
• Analysis of influenza virus types and subtypes, including age distribution shifts.
• Identification of other respiratory viruses co-circulating during the epidemic.

Main Results:

• Influenza A was the predominant virus (28.3% of sentinel samples), with a shift towards increased infections in children and young adults.
• The majority of influenza A isolates were A/Fujian/411/2002-like, a variant of the vaccine strain A/Moscow.
• No significant increase in influenza risk was observed in vaccinated populations, including the elderly, despite the vaccine mismatch.
• Rhinoviruses and human metapneumovirus were the most common non-influenza respiratory viruses identified.

Conclusions:

• The combination of influenza detections and increased influenza-like illness (ILI) accurately predicted the epidemic's onset.
• The 2003 influenza vaccine, despite a mismatch with circulating strains, appeared to offer protection, with no increased risk in vaccinated groups.
• Other respiratory viruses like rhinoviruses and human metapneumovirus also contributed to ILI during the epidemic.