(This isn't exactly equivalent but a good approximation for typical values of. If the number of new cases has decreased by 3% since yesterday, then the growth rate is, approximately, = -0.03 per day. What is needed to keep the epidemic under control is for the growth rate to be negative and hence the number of new cases to be decreasing. If the growth rate is positive, the number of new cases each day is increasing, if the growth rate is 0, the number of new cases stays constant. You can use the interactivity below to explore how the progression of a diseases changes for different growth rates (use the slider to change the value of ).ĭuring the COVID-19 pandemic the number of new cases, and new deaths, is reported each day – we watched in horror as these increased in the UK in March and early April, and have now seen them decrease over recent weeks. This leads to wildly different outcomes over the next month despite both diseases in this example having the same reproduction ratio. ![]() (The number is a mathematical constant approximately equal to 2.719 and intimately connected to exponential growth.)įor the example curves above, the growth rate for HIV is =0.002 per day and for measles is =0.06 per day. ![]() Here is the number of cases, which depends on time measured in days, and (pronounced "lambda") is what is called the growth rate of the disease per day. The growth of cases of a disease is modelled using an exponential curve: The growth rate of a disease is a natural way to capture how quickly the number of infections are changing day by day. The difference is the time between new infections – several months for HIV but just days for measles. Here are two example curves for the growth of infection, both with a reproduction ratio R=2. However for influenza or measles, where the infection is much faster, on the scale of days, R=2 means very rapid growth. For diseases like HIV or TB, where there can be months or years between one person infecting the next person, even R=2 means slow growth over time. As we saw in previous articles, R helps us understand what is happening with the disease: R>1 means that the epidemic will grow, R=1 means we are plateauing, R1), but we cannot tell how quickly. That's the number of people infected, on average, by a single infected person. ![]() We have all become used to thinking about R, the reproduction ratio (or sometimes called the reproduction number) of a disease. See here for all our coverage of the COVID-19 pandemic.
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