Why Kenyan airport fever checks can miss Ebola, and why contacts are watched for 21 days

A fever check at an airport can miss Ebola because a thermometer reads one sign, in one person, at one moment. A traveller who is infected may not have a fever yet, or medicine may be masking it. Contacts of a patient are watched for 21 days because the World Health Organization (WHO) gives 21 days as the upper end of Ebola's incubation period, the time between infection and the first symptoms.

Kenya met the problem this week. Patrick Amoth, the Ministry of Health's director-general, said the country's first confirmed Ebola patient may have passed through airport screening because medication had masked his symptoms, Reuters reported on 6 October. Health Minister Aden Duale told a news conference, as quoted by Reuters, that the Kenyan patient, who had lived in the Democratic Republic of the Congo (DRC) for seven years, had been ill for about a month before travelling by road from the DRC to Kampala and then flying to Nairobi on Saturday, 3 October. Uganda's health ministry said screening teams observed that he had a normal temperature at Entebbe airport. In Nairobi he passed health screening and was taken to Nairobi Hospital, where he was admitted with fever, chills and bleeding beneath the skin, Duale said.

WHO Director-General Tedros Adhanom Ghebreyesus said at a media briefing on 7 October that the man "received care but died on Monday this week".

The WHO fact sheet on Ebola disease, last updated on 24 April 2025, says the incubation period "varies from 2 to 21 days". It adds: "People cannot transmit the disease before they have symptoms, and they remain infectious as long as their blood contains the virus." A traveller who is infected but still incubating has no fever for a scanner to catch and, on WHO's description, cannot yet pass the virus on.

Fever is also an early sign of many illnesses. WHO lists it among symptoms that "can be sudden", together with fatigue, malaise, muscle pain, headache and sore throat. The fact sheet says "it can be difficult to clinically distinguish Ebola disease from other infectious diseases such as malaria, typhoid fever, shigellosis, meningitis and other viral haemorrhagic fevers". A scanner can flag a fever. It cannot say what caused it.

Data from the West Africa outbreak support the 21-day figure for most cases. In a 2014 paper in PLoS Currents, "On the Quarantine Period for Ebola Virus", Charles N. Haas of Drexel University in Philadelphia examined earlier estimates of Ebola's incubation period. He cited the WHO Response Team's analysis of the first nine months of the West Africa outbreak, which covered 4,010 confirmed and probable cases with usable data. The team reported a mean incubation period of 11.4 days "with an upper 95th percentile of 21 days". In other words, 95 of every 100 cases in that data fell ill within 21 days.

That leaves a small share of cases beyond 21 days. Haas wrote that "a reconsideration is in order and that 21 days may not be sufficiently protective to public health". His abstract describes 21 days as the period that "has been regarded as the appropriate quarantine period for holding individuals potentially exposed to Ebola Virus (EV) to reduce risk of contagion". WHO's fact sheet, updated in April 2025, still uses 21 days.

Contact tracing is the other half of the method. The fact sheet lists "identifying people who may have been in contact with someone infected with Ebola disease and monitoring their health for 21 days" among containment measures. Tedros said on 7 October that Kenyan authorities were monitoring 55 contacts, "including passengers and crew who were with the patient on the flight from Kampala to Nairobi". On 8 October, at a briefing for diplomats convened by Kenya's foreign ministry, Duale said 57 contacts, including passengers, crew and other people linked to the aircraft, had been accounted for and placed under monitoring, the ministry reported.

Tedros gave the figures for the DRC in the same briefing: "The number of confirmed cases has now reached 8665, with 4178 confirmed deaths." He said WHO advises against blanket travel or trade restrictions, "which are less effective than screening, contact tracing and other measures".

He said contact tracing in the DRC had "improved significantly", with around 2,000 alerts reported every day. "The majority of deaths still happen in the community instead of in treatment centres, and contact tracing is still not at the level needed," he said.