Why Kenya needs 42 days without an Ebola case, and what 21-day monitoring means

Kenya can only be declared Ebola-free after 42 days with no new case, because 42 is twice the longest time the virus can take to make an infected person ill. The 21-day monitoring is the shorter clock inside it: every person who was in contact with a patient is watched for 21 days, the outer limit of that incubation period (the gap between catching the virus and falling sick).

Health Cabinet Secretary Aden Duale spoke about both periods in a Citizen TV interview on 25 October, reported the next day by the Kenyan broadcaster Capital FM and the news site Kenyans.co.ke. "Once we have 42 days and we have nobody, Kenya will be declared Ebola-free," he said.

The 21 comes from the biology. The World Health Organization (WHO) says in its Ebola fact sheet that the incubation period "varies from 2 to 21 days". It also says people cannot transmit the disease before they have symptoms. A contact who has no symptoms is therefore not passing the virus on, but cannot be cleared until the 21 days are over.

The fact sheet lists following up contacts for 21 days as part of containment. In Kenya, Duale said people in quarantine are tested every 48 hours. Those who finish the 21-day period undergo final tests and clinical assessments, and contacts who test negative are "safe to go and join their families", he said. The government says its contact tracing, the work of finding everyone a patient met, is at 100 per cent.

The 42 comes from doubling that limit. In announcing the end of Uganda's outbreak, the WHO Regional Office for Africa described 42 days as "twice the upper limit of the incubation period" and as "the internationally established benchmark" for judging that transmission has ended. It said the period gives an extra safeguard that no chains of transmission (one person infecting the next) linked to the last case have been missed. Under WHO's rule, a country can be declared Ebola-free only after 42 consecutive days, and the countdown is tied to the last confirmed case testing negative twice and being discharged.

Uganda's count began when its last patient, an imported case, was discharged from care on 16 July.

Uganda announced the interruption of local transmission on 28 July and declared its outbreak over on 27 August. Surveillance continued after the last patient left care, and the extra monitoring confirmed no further transmission. Uganda reported 20 confirmed cases of Ebola disease caused by Bundibugyo virus: 15 imported from the Democratic Republic of Congo (DRC) and five acquired locally among contacts and health workers. Eighteen people recovered and two died. More than 800 contacts were identified and monitored.

WHO's fact sheet puts the average Ebola disease case fatality rate at around 50 per cent. There is no approved vaccine or treatment for other Ebola diseases, such as Sudan virus disease or Bundibugyo virus disease. Candidate products are in development.

The same fact sheet notes that transmission through infected semen has been documented up to 15 months after clinical recovery.

For people in Kenya, the two clocks work differently. A contact who completes 21 days and tests negative goes home. The country itself stays in the outbreak until 42 days pass. Duale said hundreds of people had been tested in the previous 48 hours and all the samples were negative.