Maeve Cullinan Global Health Security Reporter

Vital information about the rare Ebola species spreading in the Democratic Republic of Congo is now becoming established, top epidemiologists studying the virus have said.

The outbreak has infected at least 800 people and is centred in Ituri, a conflict-ridden province in eastern Congo, where health services are poor and misinformation is significantly hampering the ability of authorities to get a grip on spiralling case numbers.

Relatively little was known about the Bundibugyo species of Ebola before the current outbreak.

It has only caused two other outbreaks – one in Uganda in 2007, and another five years later in the DRC. Those outbreaks caused 149 and 59 cases respectively, and died out relatively quickly.

“We had some information from the prior outbreaks, but they happened a long time ago, and they were not nearly as large as this one. We have a lot to learn,” said Dr Krutika Kuppalli, associate professor in the division of infectious diseases at the University of Texas Southwestern and former WHO officer.

Now, a little over a month after the Congolese health ministry declared the outbreak on May 15, scientists say that important clues about the epidemiology of this rare species are steadily emerging.

This is what we know so far.

Case fatality rate: how deadly is Bundibugyo?

So far, Bundibugyo’s case fatality rate (CFR) – the proportion of people with the virus who have died – stands at 20.1 per cent.

That means just over a fifth of people who have contracted the virus have died of it.

According to the most recent data, released by the Congolese Health Ministry on Sunday, 801 cases have been confirmed, with 183 deaths.

From this, it appears that Bundibugyo is less deadly than more common Ebola species, including Zaire, the type responsible for the 2014-2016 West Africa outbreak that killed 11,000 people and most major Ebola epidemics since the disease first emerged in 1976.

Zaire Ebola is fatal in 75-79 per cent of cases.

However, experts at Airfinity, the bio-risk intelligence firm, warned that the CFR in this context – where data gathering and analysis is extremely challenging in war-torn Congo – cannot necessarily be translated into reliable, biological information about the virus.

“Case fatality rates are a very good metric to show how severe a virus is, but right now, since the healthcare infrastructure is so poor, the uncertainty is quite high because there are so many cases going undetected and that completely changes the CFR,” Dr Patricia Delgado, scientific director at Airfinity, told The Telegraph.

The CFR is also varying region by region dramatically.

In North Kivu, a province south of the epicentre in Ituri, the case fatality rate is significantly higher, at 59.1 per cent.

“North Kivu has a much higher fatality rate, but that is because we believe they are not detecting all the cases,” said Dr Kristan Piovera, an analyst at Airfinity.

Several labs in the province have reported shortages of testing equipment in recent weeks, leading to significant backlogs and requiring healthcare workers to send samples from sick patients to the Congo’s capital, Kinshasa, almost 2,000 miles away to confirm the presence of Ebola.

Meanwhile in neighbouring Uganda, where there are 19 confirmed cases, the CFR is much lower, at 11 per cent.

“Uganda is doing really good detection of cases and contact tracing, because they have the resources to do so,” said Dr Piovera.

“So we could potentially say this is the true CFR, but then we can’t fully rely on the data, because there might be some cases that are being missed,” she added.

How fast is the outbreak growing?

The number of infections has been growing significantly day by day. Since the outbreak was officially declared 33 days ago, case numbers have soared from zero to over 800. It is already the third largest Ebola outbreak in history.

According to data released by the WHO, cases have roughly doubled every 5-7 days from the middle to the end of May, rising to 600 cases in two weeks.

The curve somewhat flattened in June, rising from 650 to 801 from the beginning to the middle of the month.

However, experts at Airfinity cautioned this does not mean the outbreak is slowing.

Local health authorities in Ituri believe the index case of the current outbreak came in January, four months before the outbreak was officially declared, meaning the virus may have been spreading through communities undetected for a long period of time.

Numbers of suspected cases were initially higher, before the DRC health ministry revised down the figures as a backlog of tests were completed, leaving epidemiologists with a much clearer picture of what’s going on.

“There was a huge backlog of cases to be tested – more than 1,000 – that was cleared on May 31st,” said Dr Piovera of Airfinity.

“But now it seems broadly [authorities] have cleared that testing backlog since May 31st, so everything that’s now being reported since then is most likely new cases.”

Between Saturday and Sunday of last week alone, 72 infections were confirmed.

Analysis from the US Centres for Disease Control and Prevention (CDC) has found that, at this rate, the epidemic could generate more than 20,000 cases, approaching the scale of the West Africa outbreak a decade ago.

According to an Airfinity report released on Monday, the spread “remains uncontrolled” in communities across Eastern Congo.

Those involved in the outbreak response say a dangerous combination of conspiracy theories, mistrust and an already volatile security situation are all hampering response efforts.

Many people who become sick are unwilling to come forward and be treated – increasing the risk they pass on the virus – and some patients have even escaped from hospitals, choosing to return home and be cared for by traditional healers.

The result is that contact tracing – vital to the containment of any virus – is extremely low, at just 56.6 per cent. The WHO says at least 90-95 per cent of contacts must be traced to contain an outbreak.

The replication rate or ‘R’ number

The R-number, or basic reproduction number, is a figure used by epidemiologists to determine how contagious a pathogen is.

It describes how many people a person infected with a disease will pass it on to, but it is highly dependent on context and rarely reflects the biological characteristics of a pathogen alone.

It is largely shaped by the circumstances in which a disease spreads, including how closely a population lives to each other, healthcare standards, cultural practices, and the availability of interventions like vaccines and treatments.

Although Ebola is considered an extremely dangerous disease because of its high fatality rate, its R-number is generally lower than that of viruses that spread through the air, like Covid-19 or measles.

Measles, for example, has one of the highest R-numbers of any disease, with each infected person capable of passing the virus to between 12 and 18 people (if they are unvaccinated).

In the first two years of the pandemic, the R-number of Covid-19 ranged between two and 11 depending on circumstances. In places like cruise ships and care homes, the virus would spread faster because of close living quarters, shared ventilation, and prolonged interactions, for example.

But Ebola is much less transmissible.

The virus typically infects between one and two additional people because it spreads through contact with the bodily fluids of a sick person, rather than through tiny particles suspended in the air, as is the case with measles.

However, Ebola’s transmission rate can increase dramatically under certain conditions.

During previous outbreaks in Africa, R-numbers have been estimated as high as 10 in places with poor healthcare, inadequate infection-control measures, and close living conditions.

In the Congo, unsafe burial practices, in which family members wash and prepare the bodies of deceased relatives, have been a major driver of transmission.

Ebola victims can remain highly infectious after death, with traditional burial rituals easily becoming super spreader events under the right circumstances.

Investigators suspect that one of the first super spreader events of the current outbreak came when a pastor’s coffin broke on the rocky road home to his burial ceremony in Mongbwalu.

Scientists say they do not yet have enough data to determine the effective reproduction number for Bundibugyo.

“It’s hard to distinguish what the R-number is right now, given so many unknowns in the outbreak, but it will be something that is important to understand more as we go forward,” Dr Kuppalli told The Telegraph.

The WHO has now sent a team of epidemiologists to the outbreak zone to better understand how the Bundibugyo virus is behaving and learn how best to curb the outbreak.

Vaccines are not yet available for this species, although efforts to produce them have been significantly ramped up in recent weeks. AstraZeneca, Moderna, and Merck all have candidates moving through clinical trials, for example.

The UK Health Security Agency (UKHSA) on Tuesday announced it will send seven British specialists to the region to assist containment efforts.

“The UK Public Health Rapid Support Team brings world-class expertise, from infection control to community engagement and modelling the data. It demonstrates our commitment to global health security, and supporting the African-led response to this crisis,” said Jenny Chapman, Minister for Africa and International Development.

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