One of the three west African countries hardest hit by the raging epidemic that has killed nearly 3,900 people, Sierra Leone launched a nationwide three-day shutdown last month in a bid to find unreported cases.
   
But a team of scientists at ETHZ has determined through sequencing of early cases in the country that the average incubation period of the virus is around five days.
   
This means Sierra Leone, which turned up hundreds of new cases thanks to the shutdown, could have discovered many more if it had extended the quarantine for a few more days.
   
"You might happily stay home for three days but then on day four the disease breaks out when you actually leave the house again, and then you would transmit," research team head Tanja Stadler told AFP.
   
While the average incubation period for Ebola is five days, a person can carry the virus without showing symptoms or being contagious for anywhere from two to 23 days, she pointed out.
   
It remains unclear if nationwide shutdowns are effective at all, but if countries do opt for that route they should take guidance from the average incubation period, Stadler said.

How many missing links?

Her team, which works out of an ETHZ lab in the Swiss city of Basel, genetically sequenced 72 people infected with Ebola in Sierra Leone in May and June to build a virus family tree in a bid to reveal more about its transmission path.
   
Since Ebola mutates slightly each time it is transmitted to a new person, the scientists looked at how similar the virus was in different patients to evaluate how many missing links, or unreported cases, there were in their reconstituted transmission chain.
   
Their findings, published in the scientific journal PLOS earlier this week, show that each infected person on average transmitted Ebola to 2.18 others and that around 30 percent of cases had gone undetected.
   
Devastatingly, the data clearly shows that Ebola spread constantly through June, meaning that "whatever public health interventions were performed in Sierra Leone at that time did not help," Stadler said.
   
Sierra Leone in June counted only a few hundred infections and around 50 deaths, and it remains unclear if the findings can help inform the country's response today, as infections soar towards 2,800, with nearly 900 people already dead.
   
"Unfortunately, we don't have data from after the shutdown, because the method we developed would allow us to quantify if during (or following) the shutdown the transmission actually decreased," Stadler said.
   
Accessing Ebola data is complicated, since only a few dozen labs around the world have the security clearance to work with the deadly pathogen, and because it is dangerous for people on the ground to handle and send out the blood samples.
   
At least five of the doctors and nurses who helped gather the samples the Swiss study is based on have since died of Ebola, Stadler pointed out.
   
But she stressed that if armed with new data her team's method could quickly and quite accurately calculate the number of undetected cases in a given area.
   
"Hopefully people now know they should forward us new data if they get it," she said, adding that she hoped the research might inspire other labs to "dig deeper with other data sets."