Measles

Measles is caused by the measles (rubeola) virus, a member of the Paramyxoviridae family, and remains one of the most highly communicable infectious diseases known, with a secondary attack rate exceeding 90% among susceptible household contacts.^[1] It spreads through the air via inhalation of infectious respiratory particles — which can remain suspended for up to 2 hours after an infected person has left a space — and through direct contact with infectious nasal or throat secretions; less commonly it spreads via contaminated surfaces or objects.^[1] The illness follows a genuinely distinctive timeline: a several-day prodrome of fever, cough, coryza, and conjunctivitis, the pathognomonic enanthem of Koplik spots appearing 2-3 days after symptom onset, and finally a generalized maculopapular rash that begins on the face and spreads cephalocaudally to the trunk and limbs.^[1]

Measles is nationally notifiable in Canada, and health care providers are required to report suspected cases to their local public health authority on an urgent basis — without waiting for laboratory confirmation.^[1] There is no specific antiviral treatment; management is entirely supportive, with vitamin A supplementation considered for children with acute measles per World Health Organization guidance. The real clinical priority is preventing transmission through airborne precautions and prompt isolation, and identifying complications early, since the disease carries genuine risk of otitis media, pneumonia, encephalitis, and — rarely, but devastatingly — the fatal, delayed complication of subacute sclerosing panencephalitis (SSPE).^[1]

Incubation period and the phases of illness

The incubation period from exposure to the onset of prodromal symptoms averages about 10 days, ranging from 7 to 18 days; the interval from exposure to rash onset averages 14 days and can extend as late as 21 days.^[1][3]

^[1]

Characteristic measles rash on the third day of eruption. (CDC Public Health Image Library #4497, public domain, via Wikimedia Commons)

Koplik spots on the buccal mucosa, photographed on the third pre-eruptive day — the pathognomonic sign that classically precedes the rash. (CDC Public Health Image Library #6111, public domain, via Wikimedia Commons)

Atypical presentations — do not let a mild illness or an absent rash lower suspicion

Differential diagnosis

• Unvaccinated or incompletely vaccinated status is the dominant risk factor — among Canadian measles cases reported between 2015 and 2024 with known vaccination status, the large majority (72%) were unvaccinated.^[4]
• Age under 5 years, and particularly infants under 12 months — Canadian surveillance from this same period found the highest measles rate was in infants under 1 year of age (0.8 cases per 100,000).^[4]
• International travel to, or contact with a traveller returning from, a region with ongoing measles transmission — most Canadian cases and outbreaks originate this way, since Canada has no endemic transmission of its own outside of active outbreak periods.^[1]
• Household or other close, prolonged contact with a case — measles transmission occurs more frequently after exposures at close distance or for a prolonged period, such as sharing a home or a classroom, though transient or more distant exposures can also transmit the virus.^[3]
• Healthcare, childcare, school, congregate living, or workplace settings during an active outbreak, given the ease of airborne spread in enclosed indoor spaces.^[3]
• Pregnancy — with distinct maternal and fetal complication risks described in Special Populations.^[4]
• Immunocompromising conditions — both a risk factor for infection in the unimmunized or under-protected, and a risk factor for atypical, more severe disease if infection occurs.^[1]
• Living in a community or region where vaccination coverage has fallen below the roughly 95% threshold needed for herd immunity — this is explicitly identified as a current risk in parts of Canada despite generally high national coverage.^[1]
• Birth year and country of birth inform, but do not guarantee, presumed natural immunity: in Canada, those born before 1970 are generally presumed immune due to high measles circulation before routine vaccination, though this cutoff differs by country (1957 in the United States) and does not apply to individuals in higher-risk roles such as healthcare workers, who should have documented 2-dose immunity regardless of birth year.^[1][4]

National case classification

Measles case status is defined by the following classification, applied by public health following notification.

^[5] The clinical illness required for these definitions is fever together with at least one of cough, coryza, or conjunctivitis, plus a generalized maculopapular rash; clinical judgment should be applied more flexibly for breakthrough or immunocompromised cases, who may not present this classically.^[5]

Laboratory testing — collect specimens promptly, but never let testing delay reporting or isolation

When measles is suspected, collect a nasopharyngeal or throat swab for PCR viral detection, a urine specimen for PCR, and blood for serology as soon as possible — viral detection supports diagnosis, genotyping to help trace the source of infection, and the surveillance needed to monitor measles elimination status.^[1] IgM serology can be falsely negative if collected less than 3 days after rash onset; if an early specimen tests negative and suspicion remains, a second serum specimen collected more than 3 days after rash onset should be sent for retesting.^[5] Serologic testing is not recommended to check susceptibility before vaccination or response after vaccination.^[1] Recent MMR immunization can itself cause fever with or without a mild rash, typically 6-23 days after the dose — vaccine strain can be distinguished from wild-type virus by genotyping, and vaccination history should be considered when interpreting an ambiguous presentation.^[5]

Reporting is not optional and does not wait for lab confirmation

Measles is nationally notifiable. Health care providers are required to report suspected cases to their local public health authority on an urgent basis and should not wait for laboratory results before doing so — specimen collection and reporting proceed in parallel.^[1]

• Prevent onward transmission through prompt isolation, airborne precautions, and rapid public health reporting — the single most protective action available once measles is suspected, given how efficiently this virus spreads.^[1]
• Provide effective supportive symptomatic care, since no specific antiviral treatment for measles exists.^[1]
• Identify vitamin-A-eligible patients and dose correctly per World Health Organization guidance, to reduce the severity of complications and mortality risk.^[1][6]
• Detect and manage complications early — otitis media, pneumonia, encephalitis, and dehydration — rather than assuming a uniformly mild course.
• Deliver time-critical post-exposure prophylaxis (MMR vaccine within 72 hours, or immunoglobulin within 6 days) to eligible susceptible contacts, matched correctly to age, pregnancy, and immunocompromise status.^[4]
• Support routine and catch-up MMR vaccination to close immunity gaps, and correct common counselling misconceptions, particularly during active outbreak periods.
• Meet public-health reporting and contact-management obligations promptly and completely, rather than treating them as optional once a diagnosis feels clinically obvious.

Post-exposure prophylaxis for susceptible contacts — genuinely time-critical

For a contact who does not meet the criteria for expected measles immunity (see Risk Factors), the correct product and timing depend on time since exposure and the contact's age, pregnancy status, and degree of immunocompromise. A measles-containing vaccine is expected to be effective when given within 72 hours of exposure; immunoglobulin (Ig) can be given up to 6 days after exposure and is used where vaccine is not appropriate or the 72-hour window has passed for someone at meaningfully higher risk. MMR vaccine and immunoglobulin should never be given at the same visit — Ig blunts the vaccine's immune response.^[4]

^[4]

Isolation of confirmed or suspected cases

• Isolate immediately using airborne precautions once measles is suspected — this is the single highest-value action available, given the virus's extraordinarily high transmissibility.^[1]
• Home isolation for a confirmed or strongly suspected case should continue for 4 days after rash onset (with the first day of rash counted as day zero); for a person who does not develop a rash, isolate for 10 days after first symptom onset.^[1]
• Immunocompromised patients, with or without a rash, may have prolonged viral shedding and can remain contagious for the entire duration of their illness — extend isolation accordingly on public health direction rather than applying the standard timeline automatically.^[1]

Airborne precautions in healthcare and pharmacy settings

• Use an airborne infection isolation room (a negative-pressure room) where available, and a fit-tested N95 (or equivalent or higher) respirator for anyone caring for a patient with suspected or confirmed measles — regardless of that health care worker's own immunization or immunity status.^[1]
• Only staff with presumptive immunity should enter the room of a patient with suspected or confirmed measles where this can be arranged.^[1]
• Call ahead before a suspected case arrives at a pharmacy, clinic, or emergency department, so airborne precautions can be set up before the patient enters a shared waiting area — this is exactly how airborne outbreaks propagate in healthcare settings when skipped.^[1]

Contact management

• A contact is defined as anyone who shared a space with a probable or confirmed case during that case's infectious period, or who entered an enclosed indoor space within 2 hours after the case left it.^[3]
• Contacts who do not meet criteria for expected immunity should self-monitor for symptoms from 5 days after their first exposure to 21 days after their last exposure to the case, regardless of whether they received post-exposure prophylaxis, and should isolate immediately and contact their health care provider or local public health authority if symptoms develop.^[3]
• At public health's discretion, these same contacts may be excluded from daycare, school, or work for that same window (5 days after first exposure to 21 days after last exposure), or until 4 days after rash onset if they go on to develop measles.^[3]
• Advise contacts who do not meet expected-immunity criteria to avoid contact with others who also lack expected immunity or who are at high risk of complications, and to wear a well-fitting mask in shared indoor spaces or when accessing public spaces for essential reasons such as seeking medical care.^[3]

General supportive care

• Adequate fluids and rest; a cool, dim environment can help with the photophobia that often accompanies measles-related conjunctivitis.
• Reinforce hand hygiene, though airborne transmission — not surfaces — is this virus's dominant transmission route.^[1]
• Support timely public health contact tracing, including providing accurate information about the case's own recent contacts and locations.

Infants under 12 months

This is the age group with the narrowest post-exposure prophylaxis options and, per recent Canadian surveillance, the highest measles rate of any age group (0.8 cases per 100,000 among infants under 1 year).^[4] Infants under 6 months are not candidates for MMR vaccine — placentally transferred maternal antibody wanes over the first several months of life, and vaccine effectiveness/safety has not been established this young, so immunoglobulin is the only PEP option.^[4] During an active outbreak, MMR vaccine may be given as early as 6 months of age; if given between 6 and under 12 months, 2 further doses after the child turns 12 months old (at least 4 weeks apart) are still required for durable, long-term protection.^[1]

Pregnancy

MMR vaccine is generally contraindicated in pregnancy given a theoretical risk to the fetus, though inadvertent immunization is not itself grounds for recommending termination of pregnancy.^[4] For a pregnant contact who is unvaccinated or known to be measles-IgG-negative, IVIg 400 mg/kg IV within 6 days of exposure is the recommended strategy, with MMR vaccination deferred to the postpartum period.^[4] Measles infection during pregnancy carries real risk: pneumonia, hepatitis, and death for the pregnant person, and low birth weight, premature labour, spontaneous abortion, or congenital measles (which can itself cause pneumonia, encephalitis, and death in the infant).^[1]

Immunocompromised patients

Current Canadian guidance stratifies immunocompromised contacts into three risk tiers rather than treating "immunocompromised" as a single category. Individuals with an absent or near-absent immune system — for example within 12 months of allogeneic HSCT or CAR-T therapy, active acute lymphoblastic leukemia treatment, HIV with a CD4 count under 200 cells/mm3, or a significant primary immunodeficiency — are treated as susceptible to measles regardless of prior vaccination history, and should receive Ig-based PEP within 6 days of any exposure without waiting on serology.^[4] Those with a more moderate degree of immunocompromise may retain measles antibody-mediated protection from earlier vaccination or infection; specialist consultation and rapid serologic testing (where results are available within 24 hours) can help target Ig appropriately in this group.^[4] Those with only low-level immunocompromise, for whom a measles-containing vaccine is not contraindicated, are managed using the same expected-immunity criteria as immunocompetent contacts. Clinically, immunocompromised patients who do become infected often have more severe disease and may present without a rash at all, or with an atypical rash — a genuinely dangerous pattern to miss — and may remain contagious for the full duration of their illness rather than the standard 4-day post-rash window.^[1]

Healthcare workers

A fit-tested N95 (or equivalent or higher) respirator is required when caring for a patient with suspected or confirmed measles regardless of the health care worker's own immunization or immunity status, and only staff with presumptive immunity should enter the patient's room where this can be arranged.^[1] Given their elevated occupational exposure risk and their potential to spread infection to vulnerable patients, health care workers should maintain documented 2-dose MMR immunity (or other evidence of expected immunity) regardless of birth year, rather than relying on the general pre-1970 presumption used for the broader Canadian population.^[1][4]

Travellers and newcomers to Canada

Presumptive immunity based on birth year is country-specific: the pre-1970 cutoff applies within Canada, but a 1957 cutoff applies for those born in the United States, and many other countries achieved measles elimination after 1970 or have not yet achieved it at all, so country of birth should be considered when this information is available.^[4] Recommend catch-up MMR vaccination before travel to any region with ongoing measles transmission, and for anyone newly arrived in Canada with an uncertain or incomplete immunization record.^[1]

• Confirmed or suspected cases: monitor through the full infectious period — 4 days after rash onset, or 10 days after symptom onset if no rash develops — watching specifically for the respiratory and neurologic red flags described above; immunocompromised patients may remain contagious for the entire illness and need individualized public-health direction on when isolation can end.^[1]
• Contacts: self-monitor for symptoms from 5 days after first exposure to 21 days after last exposure to the case, regardless of whether post-exposure prophylaxis was given; public health may also conduct active monitoring when feasible.^[3]
• After immunoglobulin-based PEP: routine MMR vaccination must still be completed, but only after the correct deferral interval — 6 months after standard IMIg (0.5 mL/kg), or 8 months after IVIg (400 mg/kg) — since giving MMR too soon after Ig blunts the vaccine response. Document and actively schedule this deferred dose rather than leaving it to the patient to remember.^[7]
• Confirm that anyone who received MMR-based PEP before 12 months of age, or who has received only 1 dose earlier in infancy, goes on to complete the full 2-dose series (at least 4 weeks apart, after 12 months of age) — a single early dose is not considered adequate for long-term protection.^[1][4]
• From a public health perspective, an outbreak is not considered concluded until at least 46 days after the rash-onset date of the last outbreak-associated case, reflecting the maximum measles incubation period plus a margin for reporting delays and undiagnosed cases — a useful context point when counselling anxious patients about when "it's really over."^[3]
• Counsel on the rare, delayed risk of SSPE — onset can occur years after the original infection, there is no current monitoring test or intervention that prevents it, and this is primarily useful as a vaccination-motivation and informed-counselling point rather than an actionable surveillance task.^[1]

1. Worsening respiratory status (increased work of breathing, tachypnea, or hypoxia) at any point in the illness: this is not a "watch and wait" finding — refer urgently for assessment of pneumonia or respiratory failure, a leading cause of measles-related death.^[1]
2. New lethargy, irritability out of proportion to the illness, seizure, or altered level of consciousness: urgent/emergent assessment for encephalitis, which occurs in roughly 1 of every 1,000 reported cases and can cause permanent neurologic sequelae.^[1]
3. Persistent ear pain/discharge, or a fever that recrudesces after initially improving: reassess for secondary bacterial otitis media or pneumonia, and treat with an appropriate antibiotic if confirmed (see Drug Tables) rather than continuing supportive care alone.^[1]
4. A contact who received post-exposure prophylaxis subsequently develops fever or a rash: still isolate and report as a suspected case. PEP meaningfully reduces the likelihood of infection but does not eliminate it, and Ig recipients specifically should not be reassured into complacency by having "already been covered."^[4]
5. A pregnant patient with confirmed exposure or infection: escalate to obstetrics and infectious disease regardless of trimester, given the maternal and fetal complication profile described in Special Populations.^[1][4]
6. Diagnostic uncertainty (an atypical rash, no rash at all in an immunocompromised patient, or an equivocal early serology result): do not close the differential prematurely. Repeat serology more than 3 days after rash onset if the first sample was drawn too early, and maintain isolation and reporting pending clarification rather than assuming a negative early IgM rules out measles.^[5]

• Waiting for laboratory confirmation before reporting a suspected case to public health — report immediately, and let specimen collection proceed in parallel rather than gating the report on lab results.^[1]
• Walking a patient with a compatible fever-and-rash history straight into a shared waiting room without calling ahead — this is exactly how airborne outbreaks propagate in healthcare and pharmacy settings.^[1]
• Assuming a single prior dose of MMR, or an uncertain/undocumented vaccination history, equals full protection — the criteria for expected immunity require 2 documented doses given at least 4 weeks apart, after 12 months of age.^[4]
• Administering MMR vaccine and immunoglobulin at the same visit, or too close together in either direction — Ig blunts the vaccine's immune response, and the correct deferral interval (6 months after IMIg, 8 months after IVIg) must be observed.^[7]
• Giving MMR vaccine to a pregnant contact, or to a severely immunocompromised contact for whom it is contraindicated, instead of the correct immunoglobulin-based strategy.^[4]
• Missing an atypical, mild, or rash-free presentation in a breakthrough or immunocompromised case — measles is not "ruled out" by the absence of a classic rash in these groups.^[1]
• Assuming every Canadian-born adult is presumptively immune — the pre-1970 birth-year presumption does not apply to anyone born in 1970 or later, and should not be relied on at all for higher-risk roles such as healthcare workers, who should have documented 2-dose immunity regardless of birth year.^[1][4]
• Reaching for antibiotics for uncomplicated measles — there is no antibacterial (or antiviral) role in a straightforward case; reserve them for a confirmed or strongly suspected secondary bacterial complication.^[1]
• Overlooking vitamin A in an eligible child, or dosing it incorrectly — it is a short, age-specific 2-dose course tied to the acute illness, not an open-ended supplement.^[6]
• Losing track of the two different "clocks" in this disease: the 21-day contact self-monitoring/exclusion window (5 days after first exposure to 21 days after last exposure) versus the much shorter 4-day isolation period that applies once a contact actually develops the rash.^[3]

How to use this table: Doses are adult doses and require patient-specific confirmation (renal/hepatic function, weight, allergy history). Pediatric dosing is weight-based per the current product monograph unless stated. Drug pricing is intentionally omitted. Select any drug name to open its complete Pharmasist Drug Database page.

Post-Exposure and Routine Immunizing Agents

Product and timing depend on time since exposure and the contact's age, pregnancy status, and degree of immunocompromise — see the Post-Exposure Prophylaxis table in Choose Initial Treatment.

Supportive Symptomatic Care

First-line management for the fever and malaise of active measles infection — there is no specific antiviral treatment.

Vitamin A Supplementation — Acute Measles (WHO-Recommended, Age-Based Dosing)

Vitamin A deficiency is linked to delayed recovery and greater measles complications; two doses on consecutive days reduced measles mortality by roughly 64% versus placebo in the trial evidence underlying this recommendation. Not a Drug Database catalogue item — dosed strictly by age.

Antibiotics — Confirmed or Strongly Suspected Secondary Bacterial Complications Only

No role in uncomplicated measles. Reserve for a confirmed or strongly suspected secondary bacterial complication such as otitis media or bacterial pneumonia.