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WHO Ebola Criteria

Information note - Updated May 2015 (*)

Definition of "zero" Ebola cases

The outbreak of Ebola Virus Disease (EVD) will be considered ended in any one of the above countries after 42 days have passed since the last confirmed case has tested negative twice for the virus on blood samples (see Note 1 below). After the 42-day period has elapsed, each country should maintain a system of heightened surveillance for a further 90 days, and ensure ongoing EVD surveillance and notification thereafter. The end of the Ebola outbreak in the West African sub-region will be declared when the 42-day period has elapsed in the last affected country.

Note 1. There are 3 possible scenarios for the last case during an Ebola outbreak ·

  • The case was isolated and was confirmed positive. He/she recovered and later tested negative by PCR on two blood samples collected by venipuncture at an interval of at least 48 hours. The 42-day count starts on the day after the day on which the second negative PCR sample was collected.
  •  The case was isolated and was confirmed positive. He/she died in the Ebola treatment unit (ETU) and safe burial was organized by the ETU. The 42-day count begins on the day following burial.
  •  The case was a contact of a confirmed Ebola case. He/she died and was buried in the community and was either a confirmed case (samples taken for laboratory testing were positive) or a probable case (samples not taken for laboratory testing). The 42-day count begins on the day following burial.

Note 2. Explanation for the 42-day criterion, with 90 additional days of heightened surveillance

Forty-two days are regarded as twice the maximum incubation period for Ebola infections. A period of 42 days since the last case tested negative twice can be expected to confirm the interruption of human-to-human transmission. Given the continuing risk of new importations of EVD into any country until the entire West African sub-region has stopped transmission, the consequences of a new emergence from an animal reservoir, and the possibility of sexual transmission or a missed transmission chain, heightened surveillance should be sustained for 90 days beyond the initial 42-day period. The considerations in this regard are as follows:

  •  While transmission of Ebola virus persists in West Africa, re-introduction of the virus through an incubating or sick patient is possible, given the existence of porous borders and frequent movements of people between countries. For this reason, careful control at the borders and strict surveillance should be continued until the entire West Africa sub-region is free of Ebola. These activities require strong cross border collaboration and effective implementation of preparedness plans.
  •  There is evidence that viable Ebola virus can persist in semen for at least 82 days after symptom onset and possibly longer than 6 months. However, the frequency of sexual transmission and the period of potential infectivity of the virus in seminal fluids remain unknown. Consequently, with thousands of survivors in West Africa, there is the possibility of Ebola re-emergence via sexual transmission beyond 42 days. Apart from human-to-human transmission, the re-introduction of Ebola virus from an animal reservoir to the human population could also occur. Of six countries that experienced an Ebola outbreak previously, two reported a second outbreak within one year (Congo, Democratic Republic of Congo), one within two years (Gabon) and one within three years (Sudan). In total, four out of six countries had a second Ebola outbreak within three years. Two countries (Uganda and Cote d'Ivoire) had no further outbreaks in the following five years.

Note 3. Importance of avoiding stigma in relation to survivors

Male and female survivors of Ebola infection are at risk of stigma. Every effort must be made to minimize stigma through appropriate social mobilization, community engagement and education. Clear messages should be developed to:

(1) make it clear that survivors have completely recovered and represent no further risk for the community in general and

(2) ensure that adequate precautions (i.e. abstinence or use of condoms) are taken by survivors until the risk of sexual transmission has disappeared (see below).

How to approach surveillance and laboratory testing

During the final 42-day period During the final 42-day period each country should: (1) ensure active case finding around confirmed cases and transmission chains, (2) implement both active and passive surveillance for EVD (e.g. through regular health facility visits and by maintaining a nationwide system of alerts and signals), (3) conduct post-mortem testing for EVD following deaths in the community, and (4) offer testing of semen samples among survivors and, for those who test positive, monthly testing thereafter until 2 negative results are obtained. 1 National authorities should also ensure appropriate screening of blood donors and products. Other potential EVD surveillance activities, depending on the circumstances and goals, could include surveys of pregnant women with no history of EVD 2 and sentinel surveillance among patients with febrile illness.

After the 42-day period has elapsed

After the 42-day period has elapsed, a combination of active and passive surveillance should be maintained for 90 days, ideally integrated with surveillance for other important epidemic-prone diseases (e.g. integrated disease surveillance and response, IDSR). Passive surveillance for EVD and other diseases should be continued indefinitely. Similarly, infection prevention and control measures and EVD preparedness plans should always be in place, and monitored in all countries previously affected by EVD. Post mortem testing and the testing of survivor semen samples should also be continued for a further 90 days in each country. Other sentinel surveillance activities for EVD can be considered depending on national circumstances and goals (e.g. to ensure confidence in the health care system).

1 Safe sexual practices (use of condoms) or abstinence should be strongly recommended to survivors.

2 For pregnant women who survive EVD and continue their pregnancies, strict clinical follow-up mechanisms should be put in place. High level infection prevention and control precautions including use of full personal protective equipment, should be implemented to manage deliveries. Newborn babies should undergo Ebola virus detection and serology tests and precautions should be used to care for the baby until the tests prove to be negative.

(*) Updated 7 May 2015 Added explanation for the 42-day criterion. Added sections on: definition of "zero" cases, approach during and after the 42-day period, and stigma in relation to Ebola survivors).

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