Picture a contact tracer at a measles household. Five people live there. One has measles. Two adults are vaccinated, three children are not. She is computing two numbers as she works the case. The household secondary attack rate among the three susceptible children. And the R0 this case might contribute to in the wider county. The numbers are related. They are not the same. They will not be used the same way.
Secondary attack rate (SAR) is the share of close contacts who catch a disease after exposure to a confirmed case. R0 is the average number of secondary cases per case in a fully susceptible, well-mixed population. SAR is what contact tracers measure on the ground. R0 is what epidemic modelers feed into disease severity scoring to estimate how a pathogen behaves at population scale. Read in isolation, either one misleads. For the population-level mechanics behind R0, see our piece on R0 and transmission dynamics.
Key Takeaways
- Secondary attack rate is calculated from a defined exposure setting (most often a household): cases among contacts divided by total contacts.
- Household SAR varies enormously: measles 80 to 90 percent unvaccinated, SARS-CoV-2 wild-type 10 to 20 percent, flu 10 to 15 percent.
- R0 averages across an entire population mixing susceptible and immune contacts. SAR is conditional on close exposure where contacts are dense.
- A low household SAR does not imply low pandemic risk. R0 in a mixed population can still drive exponential growth.
- SAR drives operational calls: post-exposure prophylaxis, school exclusion, quarantine duration, contact-tracing radius.
- When SAR and R0 disagree (low SAR, high case counts), look for transmission outside the household.
What is secondary attack rate?
Secondary attack rate is the proportion of susceptible contacts who become infected after exposure to a confirmed primary case in a defined setting. The setting is almost always specified, because SAR in a household is a different number than SAR among classroom contacts or coworkers. When you read "household SAR for measles is 90 percent," that is a statement about what happens to family members of a single index case under one roof.
SAR is a conditional probability. It tells you: given that a contact was exposed in this defined way, what was the chance of infection? That conditioning is the source of both its power and its frequent misuse.
How is SAR calculated in real outbreaks?
SAR equals the number of new cases among contacts divided by the total number of susceptible contacts, multiplied by 100. If a measles patient lives with 5 unvaccinated household members and 4 fall ill, the household SAR is 4 divided by 5, or 80 percent. The math is trivial; the hard parts are defining "contact" and identifying who was actually susceptible.
Field epidemiologists set rules before they collect data. A "household contact" might mean anyone who slept in the same dwelling for at least one night during the case's infectious period. A "close contact" outside the home might mean someone within 6 feet for 15 cumulative minutes. Susceptibility filters matter too: a sibling vaccinated against measles is excluded from the denominator, because a measured SAR should reflect transmission risk to people who could actually catch the disease.
Tertiary cases muddy the count. If two household members fall ill on day 4 and a third on day 14, the third may have been infected by the second case rather than the index case. Investigators handle this with serial-interval cutoffs, counting only cases within a plausible incubation window from the primary case as "secondary."
Why is SAR different from R0?
R0 averages new cases per case across an entire population that is fully susceptible and randomly mixing. SAR conditions on a specific, dense exposure setting where mixing is anything but random. Different settings, different numbers. Households contain repeated, high-intensity contact over many days. Populations contain a long tail of brief, low-probability encounters. The two metrics sample different parts of how a disease actually spreads.
That distinction explains some counterintuitive cases. Measles has an R0 around 12 to 18 and a household SAR near 90 percent in unvaccinated families; the two numbers are both extreme because measles transmits readily in any setting. Influenza has an R0 of 1.2 to 1.5 and a household SAR of 10 to 15 percent; both are modest. Where the two metrics diverge most sharply is in pathogens with strong setting effects, such as tuberculosis, where household SAR can be high while population R0 stays low because most infections never reach the bacterium's transmission threshold.
R0 also includes contacts who are immune. In a real population, a sick person breathes on vaccinated coworkers, recovered neighbors, and people whose schedules never bring them into contact at all. SAR strips that out by restricting the denominator to defined susceptible contacts, which is why household SAR among unvaccinated contacts is often a cleaner cross-pathogen signal than R0.
What are typical SARs for major diseases?
Household SAR is the most consistently reported variant in the literature. The numbers below summarize rough consensus ranges from outbreak investigations and meta-analyses; individual studies vary with case definition, contact criteria, and immunity status of the household.
| Disease | Household SAR (susceptible contacts) | R0 (mixing population) |
|---|---|---|
| Measles | 80 to 90 percent | 12 to 18 |
| Pertussis | ~80 percent (unvaccinated) | 12 to 17 |
| Smallpox (historical) | 50 to 60 percent | 5 to 7 |
| SARS-CoV-2 (Omicron) | 25 to 40 percent | 8 to 12 |
| SARS-CoV-2 (wild-type) | 10 to 20 percent | 2.5 to 3.5 |
| Ebola | 15 to 20 percent | 1.5 to 2.5 |
| SARS (2003) | 10 to 15 percent | 2 to 4 |
| Influenza (seasonal) | 10 to 15 percent | 1.2 to 1.5 |
Two patterns stand out. First, R0 and SAR move together for most pathogens, but the relationship is loose. Smallpox has a household SAR roughly 5x flu's, but its R0 is only 4 to 5x flu's. Second, Omicron's jump in SAR (from wild-type's 10 to 20 percent to 25 to 40 percent) tracked its jump in R0 closely, which is why household studies became an early warning system for variants of concern in 2022 and 2023.
How does SAR inform quarantine and contact tracing?
SAR is the operational metric. When a public-health agency sets a quarantine duration, decides who needs post-exposure prophylaxis, or scopes contact tracing, the underlying calculation almost always uses SAR rather than R0. R0 says how big an outbreak might get; SAR says what to do for one exposed family tonight.
Pertussis is a clean example. Household SAR among unvaccinated contacts is roughly 80 percent, so CDC guidance recommends post-exposure macrolide prophylaxis for all close contacts of a confirmed case regardless of vaccination status, because even one secondary infection in an infant is potentially fatal. Compare to seasonal flu, where household SAR of 10 to 15 percent and milder typical illness mean that universal antiviral prophylaxis is not recommended for most exposed households. The math drives the policy.
Contact tracing radius works the same way. For contact tracing, agencies size their effort to the SAR profile. High-SAR pathogens (measles, pertussis) trigger aggressive ring vaccination and active monitoring of dozens of contacts per case. Lower-SAR pathogens with shorter contagious windows (Ebola via direct contact only, with household SAR around 15 to 20 percent) allow tighter, contact-by-contact follow-up without ring vaccination of entire schools.
Where can SAR mislead you?
A low household SAR does not mean low pandemic risk. SARS-CoV-2 wild-type had a household SAR of only 10 to 20 percent. That sounds modest. Yet the virus generated the largest outbreak of the 21st century. The reason is structural. Most pandemic transmission happens outside households: in workplaces, transit, schools, restaurants, and public events. Households are dense but few. Populations have many contact settings, and a pathogen that transmits even modestly in each setting accumulates an R0 well above 1.
SAR can also be artificially low when contacts have unrecognized prior immunity, which inflates "protection" that does not exist for naïve populations. The reverse error happens with high-SAR settings that are not generalizable. SAR within an unvaccinated religious community during a measles outbreak might run above 90 percent because contacts are dense, prolonged, and uniformly susceptible. That number does not predict what happens in a vaccinated metropolitan area. A SAR is always a statement about a specific setting and a specific susceptibility profile.
FAQ
Is secondary attack rate the same as attack rate?
No. The general "attack rate" is the proportion of an at-risk population that develops disease during an outbreak (cases divided by population at risk over a defined time). Secondary attack rate is narrower: it counts only contacts of a confirmed case, and only secondary cases derived from that case. Attack rate is descriptive of an outbreak; SAR is mechanistic, telling you something about per-contact transmission probability.
Can SAR be higher than 100 percent?
No. Because SAR is cases divided by susceptible contacts, it is bounded at 100 percent. If a calculation appears to exceed 100 percent, the denominator is wrong (usually missing contacts) or tertiary cases are being miscounted as secondary. Investigators typically recheck the case-definition window and contact roster when this happens.
Why is household SAR a useful early warning for new variants?
Household studies isolate transmissibility from behavior change. Population R0 estimates shift with lockdowns, masks, and seasonality, but household SAR across a stable cohort strips most of that out. Comparing a new variant's household SAR to the previous variant's gives a fast, behaviorally clean estimate of intrinsic transmissibility change, which is how Omicron's edge was flagged within weeks in late 2021.
Does SAR depend on the age of the contacts?
Yes, sometimes substantially. Children typically have higher household SARs for respiratory pathogens because they shed more virus, have closer physical contact with caregivers, and have less mature respiratory hygiene. SARS-CoV-2 was the unusual exception during the wild-type phase, where children showed lower household SARs than adults; Omicron narrowed that gap. Stratifying SAR by contact age is standard practice in any rigorous outbreak study.
How does SAR connect to herd immunity?
The link runs through R0. SAR is one input epidemiologists use to estimate R0, which then feeds the herd immunity threshold formula (1 minus 1/R0). High household SAR among susceptibles signals that per-contact transmission is efficient, which usually implies higher R0 and a higher coverage requirement to interrupt transmission. The two metrics are not interchangeable, but they constrain each other: a pathogen with very high household SAR rarely has a low R0, and vice versa.