An experimental Ebola vaccine similar to one being developed by GlaxoSmithKline is effective for at least five weeks in lab monkeys but requires boosting with an additional vaccine to extend its protection to 10 months, according to a study published on Sunday.

The findings offer an early hint of which, if any, of the Ebola vaccines in development will prove effective, and in what form. Johnson & Johnson and NewLink Genetics are also among the firms accelerating their efforts to provide Ebola vaccines and treatments as the worst known outbreak of the virus ravages West Africa, killing more than 2,000 people.

The results of the new study suggest, for instance, that a GSK vaccine now being tested on healthy volunteers will protect against Ebola infection in the short term, but may have to be augmented for long-term protection.

The study, published in Nature Medicine, is the first to report that a vaccine regimen produced "durable immunity" against Ebola, protecting four out of four monkeys for 10 months.

The vaccine uses a chimp adenovirus, closely related to a human version that causes upper respiratory tract infections, into which scientists spliced an Ebola gene.

The adenovirus infects cells in a vaccinated animal, causing them to take up the gene and produce Ebola proteins. That primes the immune system to attack the proteins of Ebola viruses when an infection occurs.

The vaccine in the study is similar to competing vaccines being developed by GSK, which began human safety trials last Tuesday, and by J&J, which aims to start safety trials in early 2015.

A third experimental Ebola vaccine uses a different delivery system, a livestock pathogen called vesicular stomatitis virus (VSV). A version developed by the Public Health Agency of Canada and licensed to NewLink Genetics is scheduled to be tested for safety in healthy volunteers this fall. Profectus BioSciences is also developing a VSV vaccine.

DOUBLE DUTY

In the new study, led by Nancy Sullivan of the National Institute of Allergy and Infectious Diseases (NIAID), researchers sought to see whether a double dose of vaccine in a regimen called "prime-boost" would address two different clinical needs.

One is protecting against Ebola immediately, said NIAID director Dr Anthony Fauci, as when a person is heading to an area with an Ebola outbreak. In the study, an adenovirus vaccine essentially the same as GSK's does so: four of four vaccinated macaque monkeys given otherwise lethal doses of Ebola five weeks later survived and had no detectable virus in their blood. All unvaccinated monkeys died within six days.

The other need is to protect someone whose stay in an Ebola zone lasts months. Here, the adeno vaccine faltered at one dose: when eight vaccinated macaques were exposed to Ebola 10 months later, six died.

With a new group of macaques, however, the scientists followed the initial adeno vaccine eight weeks later with a booster shot using a different carrier virus, called MVA (modified vaccine Ankara) and carrying the same Ebola gene. This time, all four infected monkeys were still protected at 10 months.

The human trial of the GSK vaccine uses a single adenovirus dose: regulators require safety trials to test each element of a regimen separately, said NIAID's Fauci, so any adverse events can be traced more easily.

GSK also plans to test a two-dose prime-boost version, said spokeswoman Mary Ann Rhyne.

J&J's Ebola vaccine consists of an adenovirus prime followed by an MVA boost made by Bavarian Nordic.

"After the boost, protection is not only stronger but also longer-lasting," said J&J spokesman Daniel De Schryver.

Thomas Geisbert of the University of Texas Medical Branch works on the VSV-based Ebola vaccine being developed by Profectus. He questioned the practicality of a two-shot vaccine regimen.

"You really need a fast-acting single injection vaccine" for protecting a community during an outbreak or preparing first responders and healthcare workers, he said.

Only VSV vaccines have been shown to protect lab monkeys when given after infection with Ebola, Geisbert said: "This makes it so much more useful than any of the other vaccines. For outbreaks, it works fast."