摘要：The Kermack-McKendrick (KM) (1927) epidemiological equations are used to analyse different pandemic scenarios. The infection rate constant and the withdrawal rate constant of infected individuals for the Spanish influenza pandemic (1918–1920) are found by curve fitting the solution of the KM equations to the historical data for the number of dead and the number of infected individuals during the pandemic. Hypothetically assuming the very same parameters for hypervirulent strains of H5N1 influenza virus (with 50% lethality), our simulations reveal that the latter (without vigorous countermeasures) can infect the total world population during a period of 20–30 days, with a mortality of 50% of the total world population. The short time it takes before the pandemic is over precludes the use of new vaccines that are developed only after a pandemic has started. We examine a logistically more feasible method of achieving rapid immunization after a pandemic has started (and if prefabricated vaccines cannot be used either because they are not effective or for logistic reasons): the influence of a counter-pandemic running ahead and immunizing the population before the hypervirulent H5N1 virus attacks. We find that the counter-pandemic can significantly reduce the total death toll during a pandemic with a hypervirulent strain of H5N1 influenza virus, provided that its infectivity at a population level is much larger than for the hypervirulent virus. This can be achieved if everything possible is done to hinder geographic dispersal of the hypervirulent virus (e.g. immediate cessation of all international passenger traffic, immediate cessation of all ordinary road traffic, house quarantine) at the same time as dispersal of the ‘vaccine virus’ is deliberately facilitated by sending it around in a similar way as for a vaccine, but seeding it only in every local population and not in every person as for ordinary vaccine. From a logistic point of view this might be the only feasible method of achieving immunization of a significant proportion of the population in poor countries with poor infrastructure during the very short time interval available before the superpathogen itself normally would be expected to arrive. However, mortality caused by a counter-pandemic virus will probably be higher than during an ordinary influenza epidemic, especially among elderly and malnourished persons. Vaccination, using vaccines that have been prefabricated and stored before the start of an eventual pandemic, should therefore be the preferred method of achieving immunization whenever logistically feasible.
