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Contact tracing is a classic public health intervention. It's no easy task even during small outbreaks - people who had physical contact with someone with an infectious disease are called or visited every day by a public health worker. If they develop symptoms, they are isolated to prevent them from spreading the disease further. When a single case of MERS-CoV was imported into the United States, over 500 people were under followup.
With an outbreak as large as Ebola, the number of contacts requiring follow up is dramatic. Guinea, Sierra Leone and Liberia combined have accumulated well over 30,000 contacts, each of which needs to be followed daily for 21 days. Some of those have finished their follow up period, but many thousands have not. (Several counties in Liberia are still without vehicles for contact tracing, and I assume the situation is similar in SL and Guinea. But that's a conversation for another day.)
When we compare the total number of contacts currently under follow up to the total number of contacts seen in the last 24 hours, we see both Sierra Leone and Liberia seem to be doing well keeping up with the increasing demand. Note that these are currently followed contacts, so it does not include contacts who have completed their 21 day follow up period. Both countries have around 6,000 people actively traced.
By comparing the number of new Ebola cases to the number of new contacts under follow up, we find that each case in Sierra Leone produces a median of 6 contacts, and each case in Liberia produces a median of 3. You'll notice that the distributions have a long tail though, so some Ebola cases have 30+ contacts. The median number of contacts per case has been relatively steady over time.
So that's the good news. The bad news is that the scale of the y axis on the Contacts Traced plot obscures some worrisome trends. In both countries, it's fairly common for several hundred contacts to be 'not seen' each day. This seems to be a growing problem, which is understandable as the total number of contacts requiring follow up swells. It's worrisome.
Separate from 'contacts not seen' are 'contacts lost to follow up'. The former are simply people who the contact teams didn't have time to visit, or maybe couldn't find on that particular day. The latter are people the contact teams can't find, period. They have gone missing. We only have data on this variable for Liberia, but what we see is concerning. It's not uncommon for dozens of people to be lost to follow up each day. We don't know how many of these people eventually return to the system, but my (uninformed) guess is that it's relatively rare.
Some areas are more likely to have contacts lost to follow up than others. Montserrado County has a huge number of lost contacts, which is somewhat expected given its population size and case load. Bomi County surprises me though, because it has just 85 cumulative confirmed, suspected and probable cases, and a cumulative total of 357 contacts. That means that almost 24% of Bomi's contacts are lost to follow up.
I will say that I'm assuming here that the daily reported lost contacts are incident, not cumulative. It's possible that the Liberia Ministry of Health is reporting cumulative lost contacts, and continually revising up and down as previously lost contacts re-enter the system. If this is true then there are currently just 3 contacts in the whole country lost, which I find unlikely.
So does contact tracing work? Sierra Leone is the only country reporting on the number of contacts traced that develop symptoms, but the numbers are telling. The cumulative sum of the number of contacts found ill is over 1,000 in Sierra Leone alone. Note that I only have data starting August 16, so the actual total is likely much higher.
It's critical that each contact be monitored closely, so that they can be isolated as soon as symptoms first appear. This is how we prevent transmission. Each missed case is an opportunity for the disease to transmit, and the outbreak to grow. Imperfect contact tracing is a huge challenge for this outbreak, and one that seriously limits our ability to achieve effective control.
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