Secondary Attack Rate Calculator

Secondary Attack Rate (SAR) Calculator

Calculate the probability of infection spread among susceptible contacts.

Total cases arising from contact with the primary case.

Total exposed people minus those already immune.

Secondary Attack Rate:

0%

function calculateSAR() { var newCasesInput = document.getElementById('newCases').value; var susceptibleInput = document.getElementById('susceptibleContacts').value; var resultContainer = document.getElementById('resultContainer'); var resultText = document.getElementById('sarResult'); var interpretText = document.getElementById('interpretation'); var errorMsg = document.getElementById('errorMsg'); // Reset display resultContainer.style.display = 'none'; errorMsg.style.display = 'none'; // Validate inputs if (newCasesInput === "" || susceptibleInput === "") { errorMsg.innerHTML = "Please enter values for both fields."; errorMsg.style.display = 'block'; return; } var newCases = parseFloat(newCasesInput); var susceptible = parseFloat(susceptibleInput); // Logic Validation if (isNaN(newCases) || isNaN(susceptible)) { errorMsg.innerHTML = "Please enter valid numeric values."; errorMsg.style.display = 'block'; return; } if (susceptible <= 0) { errorMsg.innerHTML = "Susceptible contacts must be greater than zero."; errorMsg.style.display = 'block'; return; } if (newCases susceptible) { errorMsg.innerHTML = "Error: New cases cannot exceed the total number of susceptible contacts."; errorMsg.style.display = 'block'; return; } // Calculation: (New Cases / Susceptible Contacts) * 100 var sar = (newCases / susceptible) * 100; // Formatting Result resultText.innerHTML = sar.toFixed(2) + "%"; // Interpretation Logic var explanation = ""; if (sar < 10) { explanation = "This indicates a relatively low transmission efficiency in this specific setting."; } else if (sar < 40) { explanation = "This indicates moderate transmissibility."; } else { explanation = "This indicates high transmissibility (e.g., highly infectious airborne pathogens or close-contact household spread)."; } interpretText.innerHTML = explanation; // Show result resultContainer.style.display = 'block'; }

The Secondary Attack Rate (SAR) is a critical metric used in epidemiology to measure the contagiousness or transmissibility of an infectious agent. Unlike the basic reproduction number (R0), which estimates spread in a completely susceptible population at large, the SAR focuses on the spread of disease within a specific, defined group of people (such as a household, a classroom, or a barracks) who have been exposed to a primary case.

How the Secondary Attack Rate Calculator Works

This calculator determines the percentage of susceptible people who become infected after exposure to a primary case. The calculation is based on the following standard epidemiological formula:

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

Input Definitions

• Number of New Cases (Secondary Cases): This refers to the number of people who developed the disease within the incubation period following exposure to the primary patient.
• Total Susceptible Contacts: This is the total number of people in the group who were exposed and were capable of contracting the disease. Note: This number should exclude individuals who are already immune due to vaccination or prior infection.

Example Calculation

Imagine a household of 6 people. One person (the primary case) contracts influenza. The remaining 5 people live in the house. However, one of those 5 people was vaccinated last week and is considered immune. Therefore, the number of susceptible contacts is 4.

If 2 of those susceptible household members subsequently get sick, the calculation would be:

• New Cases: 2
• Susceptible Contacts: 4
• Calculation: (2 / 4) × 100 = 50%

A Secondary Attack Rate of 50% in a household setting indicates a highly contagious pathogen in close quarters.

Why is SAR Important?

Public health officials use the Secondary Attack Rate to:

1. Assess Transmissibility: It provides a real-world measure of how easily a virus spreads in closed settings.
2. Evaluate Interventions: By comparing SAR values before and after implementing measures like isolation, mask-wearing, or prophylaxis, officials can determine if these strategies are working.
3. Compare Variants: SAR helps researchers understand if a new strain of a virus is more contagious than previous strains.

Difference Between SAR and Basic Reproduction Number (R0)

While both metrics measure transmission, R0 represents the average number of people one infected person will pass the virus to in a completely susceptible population without interventions. SAR is a probability (percentage) specific to a defined group and time frame, often conditional on exposure.

Factors Influencing Secondary Attack Rate

Several variables can affect the calculated rate:

• Duration of Contact: Longer exposure times usually increase SAR.
• Environment: Poor ventilation or overcrowding increases transmission probability.
• Host Immunity: Partial immunity in the "susceptible" group can lower the observed SAR.
• Virulence: The biological ability of the pathogen to infect a host.