Secondary Attack Rate Calculator

Enter Outbreak Investigation Inputs

Use exposed contacts, exclusions, and observed secondary cases to estimate transmission within a household, school, ward, camp, or similar closed setting.

Exposed Contacts

Total close contacts exposed to primary cases.

Primary Cases

Initially infected index or source cases.

Secondary Cases

New linked infections during the follow-up period.

Immune Contacts

Previously immune contacts excluded from susceptibility.

Vaccinated Contacts

Protected contacts excluded from the susceptible pool.

Quarantined Contacts

Contacts removed from effective exposure.

Symptomatic Secondary Cases

Secondary cases with symptoms.

Asymptomatic Secondary Cases

Secondary cases without symptoms.

Hospitalized Secondary Cases

Secondary cases requiring hospitalization.

Households or Clusters

Number of investigation units or clusters.

Follow-up Days

Observation window after exposure.

Serial Interval (days)

Average interval between linked cases.

Lab-confirmed Secondary Cases

Secondary cases confirmed by testing.

Manual Susceptible Contacts

Optional override when susceptibility is already known.

Plotly Graph

Example Data Table

Cluster	Exposed Contacts	Susceptible Contacts	Secondary Cases	Secondary Attack Rate (%)
Household A	12	9	3	33.33
Household B	10	8	2	25.00
Household C	15	11	4	36.36
Ward D	20	16	5	31.25

This table shows how exposed contacts, susceptibility, and observed secondary cases combine to estimate transmission within different closed-contact settings.

Formula Used

Core formula:

Secondary Attack Rate (%) = (Secondary Cases / Susceptible Contacts) × 100

Supporting formulas:

Susceptible Contacts = Exposed Contacts − Immune Contacts − Vaccinated Contacts
Effective Exposed Contacts = Exposed Contacts − Quarantined Contacts
Effective Attack Rate (%) = (Secondary Cases / Effective Exposed Contacts) × 100
Symptomatic Share (%) = (Symptomatic Secondary / Secondary Cases) × 100
Hospitalization Share (%) = (Hospitalized Secondary / Secondary Cases) × 100
Mean Secondary per Primary = Secondary Cases / Primary Cases

Secondary attack rate measures the probability of infection among susceptible close contacts after exposure to primary cases within a defined period.

How to Use This Calculator

Enter the total number of exposed contacts.
Provide the number of primary and secondary cases.
Exclude immune and vaccinated contacts from susceptibility.
Add quarantined contacts to estimate effective exposure.
Enter symptom, hospitalization, and lab-confirmation details.
Optionally override susceptible contacts when field data already defines them.
Click the calculate button to view the results above the form.
Review the graph, export results, and compare with the example table.

Frequently Asked Questions

1. What does secondary attack rate measure?

It measures how often infection spreads among susceptible close contacts after exposure to a primary case within a defined setting and follow-up period.

2. Why exclude immune or vaccinated contacts?

They may not be truly susceptible. Excluding them improves the denominator and gives a more realistic estimate of transmission among those still at risk.

3. What is the difference between exposed and susceptible contacts?

Exposed contacts include everyone who had close contact. Susceptible contacts are the subset still biologically at risk after immunity or protection is considered.

4. Can this calculator be used for households only?

No. It can also support schools, wards, dormitories, camps, and other closed-contact investigations when exposure and follow-up periods are clearly defined.

5. Why include quarantined contacts separately?

Quarantined contacts may have reduced real exposure after isolation. Tracking them helps estimate effective exposure and compare control strategies.

6. What does the risk band mean?

The band is a simple interpretation layer. Low indicates limited spread, moderate suggests notable spread, and high signals stronger within-group transmission.

7. Can secondary attack rate exceed 100 percent?

It should not when the denominator is defined correctly. Rates above 100 percent usually indicate misclassified contacts or inconsistent case counts.

8. Is this a replacement for formal outbreak analysis?

No. It is a practical estimation tool. Full epidemiologic assessment still requires case definitions, exposure timelines, testing data, and context.