It had been over 30 years since the last case of diphtheria was seen in Guinea. So when patients began showing up six months ago with what looked like flu symptoms — fever, cough and sore throat – doctors weren't alarmed. Until the children started dying.

That's when they realized that this longtime scourge, long quashed by vaccination, was back.

As of December 2023, there have been around 25,000 cases of diphtheria in West Africa and 800 deaths. In Guinea, the cases were clustered in Siguiri, a rural prefecture in the country's northeast, and early data showed that 90% occurred in children under the age of 5.

What diphtheria does — and why it's showing up

Diphtheria is a highly contagious bacterial infection spread through direct contact with infected sores or ulcers but primarily through breathing in respiratory droplets. The bacteria then releases toxins, causing inflammation that blocks the airways; a thick mucus-like substance (called a "pseudomembrane") can form at the back of the throat.

"This can kill by suffocating the patient," says Adélard Shyaka, medical coordinator for Doctors Without Borders in Guinea. "But also the toxin moves through the body and can damage the heart, the kidneys, the nervous system." Such damage — via suffocation, myocarditis, kidney failure and nerve malfunctioning — means diphtheria is fatal in up to 50% of cases without treatment.

The disease, which was a global scourge for much of the 20th century, is also almost entirely preventable through vaccination. After the diphtheria inoculation was included on the World Health Organization's essential vaccine list in the 1970s, cases decreased dramatically worldwide. "Now, it's an almost forgotten disease," says Shyaka.

But that doesn't mean this outbreak is surprising, according to Ankur Mutreja, a global health specialist with the Cambridge Institute of Therapeutic Immunology and Infectious Disease. "Diphtheria is and has always been a disease of poverty," he emphasizes, with social unrest and poor vaccination coverage explaining most outbreaks nowadays. "It's not just the West Africa outbreak but numerous other [recent] outbreaks — after the earthquake in Haiti, after war in Syria, in Bangladesh when the Rohingyas were displaced in 2017," Mutreja says.

Guinea was particularly vulnerable because of its low diphtheria vaccination rate – only 47% in 2022, with the hardest-hit Siguiri prefecture having even lower coverage at 36%. COVID-19 disrupted routine vaccination campaigns in West Africa and was associated with an uptick in vaccine mistrust, Mutreja says. But for diphtheria and other preventable childhood illnesses, the immunization problem predated the pandemic due to supply chain difficulties, insufficient funding, and complacency, among other reasons, leaving the region vulnerable to a cluster of cases swelling into an outbreak.

Progress and setbacks

In Guinea, Doctors Without Borders says its staff has supported local health workers in addressing diphtheria. Together, they've reduced mortality at Siguiri's Center for the Treatment of Epidemics from 38% to 5% over the past few months. Patients with mild symptoms are sent home with antibiotics, while more severe cases are admitted to the hospital and treated with an antitoxin, as appropriate.

However, shortages of both vaccines and antitoxins continue to hamper a full-scale response to the diphtheria outbreak, according to Louise Ivers, an infectious disease physician and the director of the Harvard Global Health Institute. Presently, only two or three companies make the antitoxin, and each batch of 1,500 doses takes about four weeks to prepare, harvested from horse blood. "Nobody wants to make it," says Ivers, because of how rarely this antitoxin is usually needed – there were fewer than 9,000 cases globally in 2021 – and how impoverished communities facing diphtheria tend to be. "That puts it into the category of low likelihood of commercial profit."

The only sure way this outbreak ends is through vaccination, suggests Ivers, who has firsthand experience responding to diphtheria in Haiti between 2003 and 2012. However, similar market dynamics may help explain the global shortage of diphtheria vaccines. "If we can catch back up with DPT [vaccines] and diphtheria boosters and get our communities highly vaccinated," she says, "then we can prevent outbreaks."

But the scarcity of vaccines means they've only been available for patients and their close contacts in Siguiri. As diphtheria continues to spread in Guinea, Mutreja worries about growing antibiotic resistance and the spread of new variants, which could render existing antitoxins and vaccines ineffective. In fact, resistance has already been increasing, decade over decade, for the past 122 years, as Mutreja described in a 2021 study in Nature Communications. "We mustn't take our eye off the ball with diphtheria. Otherwise, we risk it becoming a major global threat again, potentially in a modified, better-adapted form," he says.

While this present surge of diphtheria cases is indeed unprecedented for West Africa, it is really a symptom of larger issues in global health, including insufficient infectious disease surveillance, poor vaccination rates and scarcity of public health resources, suggests Shyaka. He's not only worried about Guinea's current diphtheria outbreak but also what other diseases are on the horizon, including meningitis, measles and whooping cough.

"The resurgence of diphtheria is an important indicator that we are far in the red zone of outbreaks — of vaccine-preventable outbreaks."

Simar Bajaj is an American journalist who has previously written for The Atlantic, TIME, The Guardian, Washington Post and more. He is the recipient of the Foreign Press Association award for Science Story of the Year and the National Academies award for Excellence in Science Communications.