How long is pertussis (whooping cough) contagious?

Pertussis Contagious Period

Untreated pertussis patients remain contagious for over 6 weeks from symptom onset, but with appropriate antibiotic treatment, patients are generally considered non-contagious after completing 5 days of antibiotics. 1

Contagious Period by Treatment Status

Untreated Patients

• Maximum contagiousness extends beyond 6 weeks from the onset of cough in untreated individuals 2, 1
• Approximately 80-90% of untreated patients will spontaneously clear B. pertussis from the nasopharynx within 3-4 weeks, though untreated and unvaccinated infants can remain culture-positive for more than 6 weeks 2
• Peak infectiousness occurs during the catarrhal phase (first 1-2 weeks) and early paroxysmal phase, when the disease is most transmissible 1
• Attack rates among exposed, nonimmune household contacts reach 80-90%, demonstrating the highly contagious nature of this disease 1, 3

Previously Vaccinated or Infected Patients

• The infectious period is typically less than 21 days in older children and adults with previous vaccination or natural infection 1
• This shortened duration reflects residual immunity that modifies disease expression and transmission 1

With Antibiotic Treatment

• Respiratory droplet precautions should continue until 5 days of appropriate antibiotic treatment are completed, as recommended by the CDC 1, 4
• Macrolide antibiotics (azithromycin preferred) are the treatment of choice for reducing transmission 1, 4
• Treatment reduces communicability within 5 days and is recommended without regard to test results within 3 weeks of cough onset for those aged >1 year (within 6 weeks for infants <1 year) 3

Important Caveats

Potential Treatment Failures in Young Infants

• A critical caveat: Two case reports of young unvaccinated infants showed persistently positive B. pertussis PCR despite 7 days of clarithromycin therapy 5
• The generally accepted assumption that contagiousness terminates after 5 days of antibiotics should be challenged, at least in young infants 5
• This suggests that the standard 5-day rule may not apply universally to very young, unvaccinated infants 5

Clinical Implications for Isolation and Prophylaxis

Isolation Recommendations

• Hospital infection control requires respiratory droplet precautions until the 5-day antibiotic treatment threshold is met 1, 4
• For untreated patients, isolation should extend for at least 3 weeks from cough onset, and potentially longer in young infants 2

Post-Exposure Prophylaxis

• Close contacts should receive macrolide prophylaxis within 21 days of exposure, particularly household contacts and those at high risk for severe disease 1, 4
• This is critical given the 80-90% transmission rate to nonimmune household contacts 1

Practical Algorithm for Determining Contagious Period

For treated patients:

• Consider non-contagious after 5 days of appropriate macrolide antibiotics 1, 4
• Exception: Exercise caution with young unvaccinated infants who may require longer treatment or isolation 5

For untreated patients:

• Consider contagious for at least 3-4 weeks from cough onset 2
• Extend to >6 weeks for unvaccinated infants 2, 1
• Peak contagiousness is in the first 1-2 weeks 1

For previously vaccinated/infected patients without treatment:

• Consider contagious for up to 21 days from symptom onset 1