SARS, lay epidemiology, and fear.

1 describe the fears that surround severe acute respiratory syndrome (SARS). In the past few weeks, our outpatient department in Germany has received numerous calls from worried doctors of companies doing business in Asia. These calls have increased since the claim was made that within 2 years every citizen of Hong Kong will be infected with SARS. A German newspaper 2 fitted an exponential curve to the cumulative number of probable SARS cases (as reported by WHO 3), forecasting a progressively steeper increase in case numbers. Such predictions can have enormous economic repercussions, so they should be scientifically tenable; we do not believe they are. To illustrate why, we have fitted curves to the cumulative cases reported from Hong Kong between Feb 21 and April 5, 2003. Microsoft Excel calculates an R 2 of 0·98 for an exponential curve, indicating an excellent fit (figure, A). Using the function of the curve, we can predict a total of 71 583 cases 60 days later. The exponential curve matches the dynamics of an epidemic in a closed population with a high basic reproductive number R 0 , which corresponds to the average number of new cases that one infectious case is expected to produce. Under such conditions, almost everyone will become infected during the epidemic. 4 However, a linear curve can be fitted to the same data, yielding an equally impressive R 2 of 0.96, but predicting only 2410 cases 60 days later. Moreover, R 2 statistics are invalid with cumulative data because the assumption that observations are independent is violated. Making predictions early in an outbreak by fitting simple curves is dubious for another reason—doing so ignores interventions to decrease contact rate and transmission probability. If successful, such efforts reduce R 0 and thereby case numbers. 4 An emerging herd immunity would also reduce R 0. This effect might only be temporary, however, since antibody levels diminish rapidly in other coronavirus infections. 5 Part B of the figure shows an alternative way of presenting the data, which we think is more informative. That there has not been a great increase in the average number of new cases per day in the past weeks becomes immediately apparent. Furthermore, the outbreak in a housing estate in late March is not followed by a pronounced peak of secondary cases 2–7 days later, as would be expected if R 0 was generally high. These data …