How Contagious Is Ebola? CDC Documents Compare Virus to COVID Delta Variant

Concerns over the threat of the Delta variant were raised after an internal document from the U.S. Centers for Disease Control and Prevention (CDC) revealed further details about the variant, including how it compares to Ebola.

According to the confidential CDC document published Thursday, which was obtained by The Washington Post, the Delta variant is more contagious than the virus that causes Ebola as well as viruses that cause MERS, SARS, smallpox, seasonal flu and the common cold.

What Is Ebola?

Ebola is a virus that causes Ebola virus disease (EVD). The fatal disease most commonly affects humans as well as non-human primates, such as monkeys, gorillas and chimpanzees, the CDC explains.

The World Health Organization says the virus is transmitted to humans from wild animals, such as fruit bats, porcupines and non-human primates.

The CDC says the disease has "occasional outbreaks that occur mostly on the African continent."

It is caused by an infection with a group of viruses within the genus Ebolavirus. Only four among these viruses (the Ebola, Sudan, Taï Forest and Bundibugyo viruses) have caused disease in people, according to the federal health body.

How Contagious Is Ebola?

The contagiousness of a virus can be described by its basic reproductive number (R0), an epidemiologic metric. So if a virus has an R0 value of 1, that means each infected person spreads the virus to one additional person.

According to a study published by the peer-reviewed journal Emerging Infectious Diseases (EID) in 2018, which was published on the CDC website, "R0 must be estimated, reported, and applied with great caution because this basic metric is far from simple."

The metric can be influenced by various biological, socio-behavioral and environmental factors that impact pathogen transmission. "Therefore, [it] is usually estimated with various types of complex mathematical models, which make R0 easily misrepresented, misinterpreted, and misapplied," the EID study said.

"R0 is not a biological constant for a pathogen, a rate over time, or a measure of disease severity, and R0 cannot be modified through vaccination campaigns...some R0 values reported in the scientific literature are likely obsolete," the EID study warned.

According to a study published by the New England Journal of Medicine (NEJM) in August 2016, "the basic reproduction number, R0, which holds at the start of an epidemic and before intervention," was estimated to be in the range of 1.71 to 2.02 in Guinea, Liberia and Sierra Leone. The range was reported to be similar to estimates from previous, smaller outbreaks of Ebola in central Africa (Sudan and Zaire species, 1.3 to 4.7).

The NEJM study explained: "Once again, the large variation in the size of the outbreaks across Africa appears to be governed not primarily by differences in R0 but by the period of epidemic growth and the size of the population at risk.

"In any event, the magnitude of R0 estimates for West Africa, calculated at an early stage of the epidemic, indicated that a sustained reduction in transmission of more than 50 percent (i.e., by a factor of more than 2) would be enough to eliminate infection from the human population. Even during a large epidemic, the elimination of Ebola virus from the human population is a feasible goal," the NEJM study said.

Is Ebola Airborne?

Ebola is not airborne and it is not likely to mutate to become airborne, the CDC said in an April 2015 article.

The federal body explained: "Even as Ebola mutates, like all viruses do, it would be very unusual for it to change how it is transmitted, especially when it is spreading easily through a population.

"Over the course of millions of years, viruses do sometimes mutate to change how they spread infection. For Ebola, this would require multiple mutations in the virus over a very long period of time," the CDC said.

How Does Ebola Spread?

The WHO explains Ebola spreads to humans via direct contact with the blood, secretions, organs or other bodily fluids (such as semen) of infected people as well as with surfaces and materials, such as bedding and clothing, that have been contaminated with these fluids.

The CDC adds that the virus can also spread through direct contact with infected fruit bats or non-human primates, such as apes and monkeys.

The federal body also explained: "The virus can remain in certain body fluids, including semen, of a patient who has recovered from EVD, even if they no longer have symptoms of severe illness. There is no evidence that Ebola can be spread through sex or other contact with vaginal fluids from a woman who has had Ebola."

How Did Ebola Start?

The CDC says scientists do not know where the Ebola virus came from but it was first discovered near the Ebola River back in 1976 in what is now the Democratic Republic of Congo.

It has since been infecting people "from time to time," which has caused outbreaks in several African countries, the CDC notes.

The WHO says the virus saw its earliest outbreaks in remote villages in Central Africa, near tropical rainforests. "The 2014–2016 outbreak in West Africa was the largest and most complex Ebola outbreak since the virus was first discovered in 1976. There were more cases and deaths in this outbreak than all others combined," according to the WHO.

The CDC said in 2015 that since its discovery, Ebola has been "a stable virus with a relatively constant mutation rate.

"Scientists monitoring the virus have not seen any evidence to suggest that the Ebola virus may be mutating to become more contagious or more easily spread," the CDC added.

A 3D illustration of the Ebola virus.
A 3D illustration of the Ebola virus seen in the blood. Rasi Bhadramani via iStock/Getty Images Plus

The Wider Picture

Coronavirus has infected more than 196.6 million people since it was first reported in Wuhan, China, including over 34.7 million in the U.S. More than 4.1 million people have died and over 4 billion COVID-19 vaccine doses have been administered worldwide, as of Friday, according to Johns Hopkins University.

The graphic below, produced by Statista, shows the seven-day average of COVID-19 vaccine doses administered and the share of the fully vaccinated U.S. population.

U.S. vaccine rollout slows down.