A household example
Suppose a hypothetical household investigation identifies one initial case and four eligible contacts. Two contacts later meet the secondary-case definition. The observed secondary attack rate is 2 divided by 4, or 50%.
Including the initial case in the denominator would answer a different question.
Timing does not prove the transmission link
A later household case may have shared an outside exposure or acquired infection elsewhere. Two people with close onset times may be co-primary cases rather than one infecting the other.
The investigation needs disease-specific timing and exposure information.
Report the rules
- Which contacts were included?
- How was susceptibility handled?
- What follow-up period was used?
- How were asymptomatic infections or untested contacts treated?
- Were shared external exposures considered?
These choices can change the estimate substantially.
Compare studies cautiously
Different testing schedules, household definitions, immunity patterns, and case ascertainment can produce different estimates even for the same pathogen. The percentage is not a universal probability that applies to every household.
Use it as a setting- and method-specific summary. A clear denominator and follow-up rule make the estimate more useful than a dramatic percentage presented without the investigation process that produced it.
