Single-Dose Of Ebola Vaccine May Provide Protection Against Deadly Disease for Up to 2 Years

A recently published study is offering promising signs for an Ebola virus vaccine manufactured by Merck, with researchers now finding that the vaccine induces protection from the deadly disease for 1 to 2 years.

Ebola virus is a highly contagious disease that can begin with symptoms such as fever, severe headache, and weakness. Those with Ebola can go on to develop serious symptoms including hemorrhaging, and if left untreated the disease can result in death. The virus spreads through the bodily fluids of those infected, contaminated surfaces, and infected primates or fruit bats.

The Ebola outbreak that ravaged parts of Guinea, Liberia, and Sierra Leone from 2014 to 2016 included 28,616 reported cases and resulted in 11,310 deaths. In a 2017 study, researchers found that individuals who recover from an Ebola infection can continue to produce antibodies for the virus for up to 40 years. Since the 2014 outbreak, public health officials have expressed an urgent need for an Ebola vaccine to prevent a similar outbreak and protect those in areas at high risk of Ebola.

In 2014, American pharmaceutical manufacturer Merck announced an agreement to research, develop manufacture, and distribute an investigational recombinant vesicular stomatitis virus (rVSV) vaccine expressing the Zaire Ebola virus glycoprotein (rVSV-ZEBOV), as Phase 1 clinical trials of the vaccine candidate were underway. In a Phase 1B study published in 2017, researchers reported that rVSV-ZEBOV effectively stimulated antibody production from the virus persisting for up to 1 year. Now, a new study, published in the journal Lancet Infectious Diseases, effectively increases the duration of the vaccine’s protection from Ebola to 2 years following immunization.

In the new study, an international team of researchers aimed to assess antibody persistence in African and European volunteers from 3 previous trials who had received a single dose of rVSV-ZEBOV. The original 217 study participants had received the vaccination from 2014 to 2015, with either a low dose or high dose form of the vaccine and returned for follow up in the new study. The 3 original trials took place in Switzerland, Gabon, and Kenya. At 1 year, 197 participants from all 3 locations returned to provide samples; 90 participants from Switzerland returned at 2 years. The 44 patients who had received the high dose form of the vaccine all remained seropositive at 2 years, while 33 of the 37 patients who had received the lower dose of rVSV-ZEBOV were still seropositive.

The antibody response was stable for all dose levels and trial locations for glycoprotein-specific antibodies. However, levels of neutralizing antibodies were less durable, as seropositivity declined from 64% to 71% at 28 days to 27% to 31% at 6 months in participants from the Switzerland study. Currently, researchers have yet to determine if neutralizing or glycoprotein-binding antibodies offer more protection from Ebola.

“These findings are very encouraging in that the vaccine, at low or higher doses, induces durable immune responses that may translate into durable protection,” said Beth-Ann Coller, PhD, the executive director of Vaccines Clinical Research for Merck Research Laboratories, in an interview with Contagion^®. “Studies to understand the relationship between immunogenicity and protection are ongoing with results expected next year.”

With the need for a single-dose Ebola vaccine that offers long-lasting protection, the new findings on rVSV-ZEBOV are promising, particularly considering the challenges involved with administering vaccine booster shots in regions affected by the virus.

For now, Dr. Coller notes that Merck is working on the testing and manufacturing activities needed to support licensure of the vaccine. “The exact timing for filing is still under discussion with regulatory agencies,” she explained. “In the meantime, we are maintaining a stockpile of more than 300,000 emergency-use dose equivalents that can support an outbreak response, should the need arise. Although the vaccine is not yet licensed there are mechanisms to support the use of the investigational product should an outbreak occur.”