Forty-three people who had been in contact with Ebola victim Thomas Duncan have been cleared, proving not to have the disease. That’s a big relief for health officials, though 120 people are still being monitored. Meanwhile, the World Health Organization says that the outbreaks in Senegal and Nigeria are over. How do they know that? Well, the answer is simple, as you will see after the jump.
From Alexandra Siffilin, What Does It Mean for an Ebola Outbreak to End? | TIME:
Nigeria’s most recent outbreak of Ebola is over, the nation’s government and World Health Organization (WHO) announced on Monday.
But — with fear of Ebola continuing to grip the world — what does that even mean? How does the WHO know that Nigeria is in the clear?
The answer, it turns out, is very specific: The WHO says a country can declare their outbreak to be over when it makes it through 42 days without a new case. That’s two incubation periods for the Ebola virus, so as long as 42 days have passed, during which the country had in place active surveillance and diagnostics but discovered no new cases, the WHO says it’s enough time to confidently say an outbreak is over. For health care workers to be considered “in the clear” they have to be monitored for 21 days after their last possible exposure to the virus, even if they were wearing full protective gear. Health care workers’ date of last contact is considered the day when the final patient with Ebola tests negative for the disease.
“Recent studies conducted in West Africa have demonstrated that 95% of confirmed cases have an incubation period in the range of 1 to 21 days; 98% have an incubation period that falls within the 1 to 42 day interval,” said WHO in a statement. “WHO is therefore confident that detection of no new cases, with active surveillance in place, throughout this 42-day period means that an Ebola outbreak is indeed over.”