Influenza: The elusive plague—Part 1 (Guest editorial)

Every year about one in six Canadians is infected with influenza, resulting in 1.5 million work days lost and 50,000 hospitalizations. More than 1200 people in BC die each year from pneumonia and influenza--a marker for influenza mortality that likely underestimates the true impact by four-fold. Up to two-thirds of the excess deaths due to influenza are related to cardiovascular disease, and its role in exacerbating chronic conditions is often missed in statistical summaries. More than 90% of influenza deaths occur in the elderly, yet fewer than half of elderly people avail themselves of a free vaccine that is up to 80% effective in preventing mortality due to influenza in this age group.[1] 

Many of the measures proven to combat influenza are grossly underutilized. Influenza immunization remains the cornerstone of prevention, yet uptake of this safe, effective, and free vaccine by all eligible persons remains dismally low. Recent surveys show only 10% to 15% uptake among children and young adults at high risk. Healthcare worker vaccination has been consistently low despite evidence that failure to be vaccinated places patients in their care at risk.

Amantadine is an antiviral agent that is 70% to 90% effective in preventing influenza A.[1] It was first licensed for this purpose in Canada in 1979. Since 1986, the National Advisory Committee on Immunization has annually recommended its use for the control of institutional outbreaks. Until recently, this recommendation was inconsistently applied in BC.

Influenza demonstrates a remarkable ability to adapt and it is this quality that has assured its place among the world's greatest scourges. Every couple of years, influenza develops minor modifications in its protein make-up. Such minor changes, or drifts, are responsible for annual epidemics. Influenza A is also capable of major changes, or shifts, in its surface protein composition. Such shifts are responsible for pandemics, which occur irregularly every 10 to 40 years when a novel influenza A virus emerges that is capable of rapid spread within a highly susceptible population.

The most devastating pandemic was that of 1918-19. This pandemic killed 40 million to 50 million people worldwide; the often-quoted figure of 20 million people is a substantial underestimate. It was distinguished by an ability to cause severe disease and death in young adults.[2] The most recent pandemic occurred in 1968, and experts agree that we are due for another within the next 5 to 10 years. When six deaths occurred in Hong Kong in 1997 among 18 people hospitalized with an influenza A never before identified in humans (the so-called "bird flu"), the world recognized that the catastrophe of 1918-19 could be repeated.

Contingency planning is already underway to prepare for just such an event. Response time will be short and many details must be worked out in advance. Peak morbidity in North America is expected about 5 to 7 months following the emergence of the pandemic strain and up to 50,000 Canadian deaths are expected within 6 to 12 months. This is based on historic trends but future pandemics may disseminate even more rapidly. Successful preparedness requires attention to surveillance activities, vaccine and antiviral requirements, decision-making procedures, communication lines, and community services. The most important component of pandemic preparedness, however, is enhancement of inter-pandemic surveillance and prevention measures.

The current influenza surveillance system in BC is among the best in North America. It includes report of school and long-term facility outbreaks, centralized laboratory confirmation, and a network of sentinel physicians across the province. Its intent is to track the yearly arrival and progress of influenza in BC and to collaborate with national and international surveillance centres. The physician-based sentinel surveillance network has been in place since 1976 and is a joint effort by the College of Family Physicians, the BC Medical Association, and the BC Centre for Disease Control.

Its success is due to the voluntary involvement of many physicians across the province whose commitment we gratefully acknowledge. Traditionally, the sentinel surveillance system is active from October to May, but identification of summer influenza outbreaks among cruise-ship travelers to Alaska and the Yukon in 1998 prompted year-round surveillance. Pandemics typically emerge during the spring and summer, and year-round surveillance will heighten our ability to identify unusual activity.

Vaccinating the susceptible population will be the best defence during a pandemic. Between 23 million and 60 million doses of vaccine (depending on whether one or two doses are required) will need to be administered to Canadians within a 1 to 2 month period. This figure represents five to ten times the number of doses currently delivered in annual influenza vaccination programs and far exceeds current Canadian manufacturing capacity. Encouraging high uptake of influenza vaccine each year by eligible persons will provide annual protection while it narrows the gap between current capacity and pandemic requirements. Furthermore, a single dose of pneumococcal vaccine administered to eligible persons now will provide year-to-year protection against this major bacterial complication of influenza, and protection will extend through the pandemic period.

Excess institutional outbreaks are expected during a pandemic, and antiviral prophylaxis will be a necessary part of their control. In addition, antivirals may be the only line of defence in the early stages of a pandemic before a vaccine is available. Lack of physician familiarity with antiviral availability, dosing, or side effects will retard their use during this critical period. Developing outbreak policies and protocols for antiviral administration and comfort in prescribing them not only reduces the morbidity associated with annual epidemics, it also develops the infrastructure necessary to ensure their efficient implementation during intense pandemic activity.

Interpandemic influenza activity is significant because of the huge toll it exacts--more people die from influenza in the 10 to 40 years between pandemics than will perish during the next pandemic. Optimal implementation of interpandemic control measures serves the dual purpose of reducing the yearly impact of influenza while promoting development of pandemic capacity, communication, and cooperation. In this five-article series, emphasis is placed on enhancing the use of existing surveillance and prevention measures while testing protocols that will be necessary during a pandemic.

The first article provides a review of surveillance results for the 1997-98 influenza season. This season was distinguished by the unexpected emergence of a new drift strain not included in the vaccine. The article provides a glimpse at the overwhelming impact of intense influenza activity and also describes unusual summer cruise-ship outbreaks. The second article provides a framework for influenza outbreak control and antiviral prophylaxis in institutions. The third article reviews breakthroughs in anti-influenza chemotherapeutic agents including amantadine, zanamivir, and oseltamivir. The series concludes in the next issue. The fourth article will provide a powerful argument for healthcare worker immunization. The final article describes successful attempts to increase influenza vaccination uptake among at-risk children. My thanks to each of the authors for lending their expertise in pulling this series together.

—Danuta Skowronski, MD, FRCPC, physician epidemiologist, BC Centre for Disease Control, Vancouver