How to Calculate Secondary Attack Rate

Secondary Attack Rate Calculator

The total number of people exposed to the primary case.

People exposed but not susceptible to the disease.

People who contracted the disease from the index case.

Secondary Attack Rate (SAR)

0.00%

function calculateSAR() { // Get input values var totalContacts = document.getElementById('sar_total_contacts').value; var immuneContacts = document.getElementById('sar_immune_contacts').value; var newCases = document.getElementById('sar_new_cases').value; var resultContainer = document.getElementById('sar_result_container'); var errorMsg = document.getElementById('sar_error_msg'); var resultValue = document.getElementById('sar_final_value'); var resultSummary = document.getElementById('sar_summary'); // Reset display resultContainer.style.display = "none"; errorMsg.style.display = "none"; // Validation if (totalContacts === "" || newCases === "") { errorMsg.innerHTML = "Please enter values for Total Contacts and New Cases."; errorMsg.style.display = "block"; return; } var total = parseFloat(totalContacts); var immune = immuneContacts === "" ? 0 : parseFloat(immuneContacts); var cases = parseFloat(newCases); // Logic Checks if (total < 0 || immune < 0 || cases total) { errorMsg.innerHTML = "Immune contacts cannot exceed total contacts."; errorMsg.style.display = "block"; return; } var susceptible = total – immune; if (susceptible === 0) { errorMsg.innerHTML = "No susceptible population (Total Contacts – Immune = 0). Cannot calculate rate."; errorMsg.style.display = "block"; return; } if (cases > susceptible) { errorMsg.innerHTML = "New cases cannot exceed the number of susceptible contacts."; errorMsg.style.display = "block"; return; } // Calculation: SAR = (New Cases / Susceptible Population) * 100 var sar = (cases / susceptible) * 100; // Update UI resultValue.innerHTML = sar.toFixed(2) + "%"; resultSummary.innerHTML = "Out of " + susceptible + " susceptible individuals, " + cases + " became infected."; resultContainer.style.display = "block"; }

In epidemiology, the Secondary Attack Rate (SAR) is a critical metric used to measure the contagiousness of an infectious disease. Unlike the basic reproduction number (R0), which applies to a general population, the Secondary Attack Rate focuses on the spread of disease within a closed group, such as a household, a classroom, or a dormitory, after a primary case (index case) has been introduced.

This calculator helps researchers, students, and health officials quickly determine the transmission risk within specific settings by accounting for the total contacts and the immune status of those individuals.

The Secondary Attack Rate Formula

The calculation calculates the probability that infection occurs among susceptible people within a specific group. The formula is:

SAR (%) = ( Number of New Cases / Susceptible Contacts ) × 100

To perform this calculation accurately, you must define the "Susceptible Contacts." This is calculated by taking the total number of exposed contacts and subtracting those who are already immune (due to vaccination or prior infection).

Note: The primary (index) case is excluded from the denominator because they are already infected.

Step-by-Step Calculation Example

Let's look at a practical example of how to calculate the secondary attack rate in a household setting.

• Scenario: A family of 6 people lives together.
• Index Case: One person comes home sick (the primary case).
• Total Exposed Contacts: The remaining 5 family members are exposed.
• Immunity: 1 family member is fully vaccinated and considered immune.
• Outcome: 2 of the remaining family members get sick within the incubation period.

The Math:

1. Identify the susceptible population: 5 (exposed) – 1 (immune) = 4 susceptible people.
2. Identify new cases: 2 people.
3. Apply formula: (2 / 4) × 100 = 50%.

In this scenario, the Secondary Attack Rate is 50%.

Why is SAR Important?

Calculating the Secondary Attack Rate allows epidemiologists to:

• Assess Transmissibility: High SAR values indicate a highly contagious pathogen in close-contact settings.
• Evaluate Interventions: By comparing SAR before and after implementing hygiene measures or isolation protocols, officials can see if those measures are working.
• Determine Vaccine Efficacy: Comparing SAR in vaccinated vs. unvaccinated groups helps quantify how well a vaccine prevents transmission.

Frequently Asked Questions

Does the total number of contacts include the sick person?

No. When calculating SAR, the denominator (the bottom number of the fraction) should only include those at risk of becoming sick. The index case (the person who brought the infection into the group) is excluded from the count of contacts.

Why do we subtract immune contacts?

If someone is immune, they are theoretically not part of the "population at risk." Including them in the denominator would artificially lower the attack rate, making the disease appear less contagious than it actually is to susceptible individuals.