ABUJA, Nigeria (AP) — Authorities in several West African countries are trying to manage their huge diphtheria outbreaks, including in Nigeria where a top health official said Thursday that millions are being vaccinated to cover wide gaps in immunity against the disease.

At least 573 people out of the 11,640 diagnosed with the disease in Nigeria have died since the current outbreak started in December 2022, though officials estimate the toll — now on the decline because of treatment efforts — could be much higher across states unable to detect many cases.

In other countries like Niger, 37 people had died out of the 865 cases as of October, while Guinea has reported 58 deaths out of 497 since its outbreak started in June.

“As far as the history that I am aware of, this is the largest outbreak that we have had,” Ifedayo Adetifa, head of the Nigeria Centre for Disease Control, told The Associated Press.

The highly contagious bacterial infection has been reported in 20 of Nigeria’s 36 states so far.

A major driver of the high rate of infection in the region has been a historically wide vaccination gap, the French medical organization Doctors Without Borders, or MSF, said in a statement on Tuesday.

In Nigeria, only 42% of children under 15 years old are fully protected from diphtheria, according to a government survey, while Guinea has a 47% immunization rate — both far below the 80–85% rate recommended by the World Health Organization to maintain community protection.

The fate of the affected countries is worsened by the global shortages of the diphtheria vaccine as demand has increased to respond to outbreaks, the MSF said.

“We’re not seeing vaccination happen, not at the scale that is needed,” said Dr. Dagemlidet Tesfaye Worku, emergency medical program manager for MSF in Abidjan, Ivory Coast. “What is needed is a truly massive scale-up of vaccination, as soon as possible.”

The Nigerian government is ramping up vaccination for targeted populations while assisting states to boost their capacity to detect and manage cases, said Adetifa, the Nigeria CDC head.

But several states continue to struggle, including Kano, which accounts for more than 75% of cases in Nigeria but has only two diphtheria treatment centers, according to Abubakar Labaran Yusuf, the state’s top health official.

“Once people have to travel or move significant distances to access treatment, that becomes a challenge,” Adetifa said.