With a propagated source epidemic, phases of infection occur at regular intervals. The time-period between these phases is called the serial interval (Fig. 2.5). Features of the epidemic are measured in the same way as a common source epidemic, while an estimate of time of recurrence is given by the serial interval. After several propagated epidemics, cases remaining from the previous epidemic will merge with the next so that the regular serial pattern will be lost.
Contagiousness or the probability that an exposure will lead to a transmission is measured by:
a particular event in time that brought all the cases together or linked them by a common phenomenon), then the incubation period can be calculated and a disease (or aetio-logical agent producing a disease) with this incubation period can be suspected. This method was used to work out the incubation period for the first epidemic of Ebola haem-orrhagic fever, as there were a large number of fatal cases that occurred in one hospital at the same time.
In an extended source epidemic, the time of infection can be deduced by measuring back in time from the first case on the rising epidemic curve to the maximum and minimum incubation periods of the diagnosed disease. Search within this defined period of time can elucidate the source.
Epidemics are suitably described by expressing them in attack rates. In a common source epidemic, the overall attack rate is used:
Overall attack rate Number of individuals affected during an epidemic Number (of susceptibles) exposed to the risk
= Number of cases within the period of one minimum and one maximum incubation period (secondary cases) from the primary case Number (of susceptibles) exposed to the risk
An example is smallpox, which had a high secondary attack rate and was therefore very contagious. Since smallpox was eradicated and people are no longer vaccinated, the level of immunity has waned and there is the fear that a very similar disease, monkey-pox, could now increase and be a threat. However, it has a lower secondary attack rate; so we can rest assured that this is unlikely to happen (see also Section 18.2).
Was this article helpful?