Attack Rate

Attack rate is a measure used in outbreak investigations to describe the proportion of people who become ill after being exposed to a particular infectious agent over a defined period of time.

It is usually expressed as a percentage and calculated as: Attack rate = (Number of new cases / Number of people at risk) × 100

Despite the name, attack rate is not a true rate in the epidemiologic sense—it’s a proportion. But it remains one of the most practical and intuitive tools for assessing how efficiently a disease spreads in a specific setting, especially during acute outbreaks.

Why attack rate is useful in outbreaks

Attack rate is most useful in settings where both the timing and population at risk are clearly defined: households, schools, camps, workplaces, long-term care facilities, or other closed environments.

What attack rates reveal

They help identify who was most at risk, the likely mode of transmission, and potential points of intervention.

Ways to break down attack rates

• By age group
• By vaccination status
• By type or intensity of exposure
• By household or room location
• By underlying medical conditions
• By time of exposure (e.g., meal shifts)

Common environments where attack rates are calculated

• Households
• Nursing homes
• Schools and childcare centers
• Camps and dormitories
• Cruise ships and enclosed transportation

These closed settings make it easier to define the population at risk and determine exposure windows.

Examples of how attack rates are applied

• Identifying high-risk exposures in foodborne outbreaks
• Assessing vaccine effectiveness in school outbreaks
• Tracking household transmission during pandemics

Secondary attack rates and their value

• Measure transmission from a known primary case
• Evaluate infection control within families or close groups
• Help determine contagiousness and effectiveness of isolation

Case study: Norovirus on a cruise ship

How the attack rate was calculated

If 120 out of 300 passengers who ate at a buffet became ill, the attack rate would be 40%.

• Clear exposure source: the buffet
• Defined population at risk: 300 passengers
• Outcome of interest: gastrointestinal illness

Why the attack rate mattered

The figure helped characterize the outbreak and prioritize interventions.

• Triggered food safety reviews
• Prompted enhanced surface disinfection
• Led to isolation of symptomatic individuals

Patterns observed in cruise ship outbreaks

Attack rates often vary by dining location or crew-passenger interaction.

• Buffets and shared dining spaces pose high risk
• Cabinmates often show clustered cases
• Transmission can continue without rapid isolation

Attack rate in COVID-19 settings

During COVID-19, secondary attack rates were used to understand transmission in schools and homes.

• Compared cases in masked vs. unmasked classrooms
• Helped guide school reopening policies
• Informed contact tracing priorities

Building the outbreak puzzle with attack rates

When used with incubation periods and exposure windows, attack rates clarify how an outbreak occurred.

Interpreting and communicating attack rates

• Always define the population and exposure clearly
• Pair with timelines to assess causality
• Use to support control measure decisions

Limitations and considerations

• May be underestimated if cases are asymptomatic
• Requires complete and accurate data on exposures
• Needs to be interpreted in the context of the setting

Policy and practice applications

How to use attack rates in outbreak response

• Identify high-risk exposure settings
• Tailor interventions to affected subgroups
• Estimate potential future spread within a group
• Evaluate effectiveness of mitigation strategies
• Guide communication with the public and stakeholders