The World Health Organization is “rationing” Ebola vaccines in Democratic Republic of Congo, with access controls meaning too few people at risk are being protected in an outbreak of the deadly disease, the aid group MSF said on Monday.

The medical charity Medicins Sans Frontiers (MSF) accused the WHO of using a rigid system of eligibility for vaccination, and said the restrictions are allowing the viral disease to resurge in communities previously thought to be protected.

“The WHO is rationing Ebola vaccines and hampering efforts to make them quickly available to all who are at risk of infection,” MSF said in a statement. “As a result, the outbreak keeps coming back to areas that have supposedly been covered by vaccination.”

The WHO denied it was rationing the vaccine and said it was working as hard as any organization to end Congo’s deadly Ebola outbreak.

“We partner closely with the DRC government to reach as many communities and individuals in the outbreak area as possible and are not limiting access to vaccine,” it said in a statement.

The Congo Ebola outbreak has killed more than 2,100 people since the middle of last year, second only to the 2013-16 Ebola outbreak in West Africa that killed more than 11,300.

MSF called for more transparency around access to the vaccine, which is manufactured by the U.S. drugmaker Merck & Co Inc and is being deployed in the WHO-led emergency response.

“Time is of essence in an outbreak: medical teams should be able to rapidly provide treatments or vaccines based on what they see on the ground,” MSF’s emergency coordinator, Natalie Roberts, said. “But our capacity ... is severely undermined by a rigid system which is hard to comprehend.”

The WHO and the Congolese health ministry say that since August 2018, more than 223,000 people have been vaccinated with rVSV-ZEBOV, the Merck vaccine that has been shown in clinical trials to be highly protective against Ebola infection.

MSF said it estimates that based on the number of Ebola cases in the outbreak so far, the vaccine should have been given to twice as many - between 450,000 and 600,000 people.

Congo health authorities gave the go-ahead on Saturday for plans to introduce a second Ebola vaccine, made by Johnson & Johnson, to help fight the outbreak.

The Merck shot is being deployed in a strategy known as “ring vaccination”, which aims to control Ebola by identifying and offering the vaccine to contacts of those likely to be infected.

The WHO said that because Ebola spreads via person-to-person contact, this is “the most effective means of stopping” its spread. It said the Merck vaccine eligibility and strategy were recommended by independent specialists in agreement with Congo.

The plan with the addition of the J&J vaccine, it said, is to extend protection by providing it to “targeted at-risk populations” in areas where the disease is not yet being actively transmitted.