Data on case characteristics, when available, are used to compare incidence or rates between various groups of cases, for example between different ages, between sexes or between different professions. This is useful to detect outbreaks which predominantly affect a particular group. The result of these comparisons can be expressed as a relative risk (RR) and its confidence interval.

When cases and deaths are notified, the case-fatality (CF) is often a useful indicator to compute. An increase in the case fatality may alert on an increase in the severity of the disease corresponding to a specific germ (e.g. cholera when diarrhoea is under surveillance as a syndrome).

Additional information on case characteristics, such as vaccine status, is not collected routinely since it is rarely necessary for triggering a public health alert. In most surveillance systems, this type of analysis is only performed at the "response" stage (investigation), and only on an ad hoc basis.

Analysis of case characteristics may be subject to biases as notified cases may not be representative of all cases in the population:

  • Females and young children may be overrepresented since they are more likely than adult men to seek medical care in some settings, while in other male cases may be more likely to be attending health care facilities
  • Milder cases of a condition may be underrepresented as such patients do not seek medical attention

These biases may affect the results of the analysis. Results must therefore be interpreted in the light of potential biases.