The impact of pandemics and new responses to emerging infectious diseases

Abstract

Several indicators can be used to assess the impact of a pandemic. For an epidemiologist, mortality or severe morbidity rates are a natural indicator - - and one that holds surprises. In fact, the mortality of epidemics with high media impact (e.g. : Sras, Ebola), is much lower than that of silent and much more devastating epidemics (e.g. : hepatitis B and C) : the Sras epidemic killed 774 people in six months, that of Ebola 11 500 in 18 months in West Africa, while hepatitis B and C kill more than 2 500 people every day, i.e. one million people a year. Diseases linked to the SARS and Ebola viruses share the common features of a short incubation period (one to two weeks) and a noisy clinical expression (respiratory distress or haemorrhagic fever). This makes them frightening, but also more vulnerable to control measures, as contagiousness does not begin until several days after the onset of symptoms. It is therefore possible to identify patients on the basis of their symptoms and isolate them before they become contagious.

Conversely, a virus such as hepatitis C has a long incubation period (20 to 30 years), and a silent clinical expression (patients only become symptomatic at the cirrhosis stage), allowing the virus to spread surreptitiously throughout the incubation period, when patients are already contagious. Today, over 300 million people are chronically infected with hepatitis B and C viruses. Epidemics will also have a considerable economic impact, slowing down trade and tourism, and closing airports, hospitals and even schools. The economic cost of the 2003 SARS epidemic in Hong Kong is estimated at $50 billion. For epidemics with high mortality rates, such as AIDS, the economic cost will be all the greater the more economically active the population affected by the disease. The World Bank estimates that a severe influenza epidemic, on a scale similar to the Spanish flu of 1918-1919, could kill 70 million people and have an economic cost of 7 000 billion dollars.

In the face of these emerging infectious diseases, new responses are emerging. New-generation high-throughput sequencing enables the identification of unknown pathogens without any preconceived ideas about their genomic structure, while the real-time production and comparison of viral sequences during an epidemic should enable transmission chains to be reconstructed in the near future. Recent history has taught us that we are unable to predict where and when the next epidemic will occur : it is therefore essential to have global surveillance resources enabling early detection of outbreaks of abnormal cases for immediate investigation. The revision of the International Health Regulations since 2005, obliging countries to report any abnormal epidemic event to the WHO, and the monitoring of epidemic-related key words by specialized Internet search engines, contribute to a much greater responsiveness of the international community to such events.

Finally, the development and pre-positioning for a phase 3 trial of vaccines against a list of 11 pathogens with high pandemic potential, within the framework of a public-private partnership called CEPI, is a recent initiative which completes our arsenal in response to these new threats.