Community burden of diphtheria during the 2023-24 epidemic in Kano State, Nigeria: a population-based household survey

A population-based household survey in Kano State, Nigeria, revealed that the community burden of the 2023–24 diphtheria epidemic was over four times higher than facility-based estimates, with most deaths occurring at home due to delayed care-seeking and low vaccination coverage.

The Gist

Imagine Kano State, Nigeria, as a large, bustling neighborhood. For a long time, the local health clinic has been keeping a list of people who come in sick with a specific, dangerous throat infection called diphtheria. By the end of 2023, that list had over 25,000 names. The clinic thought, "Okay, we know the size of the problem; it's big, but we have a handle on it."

But here's the twist: The clinic was only seeing the tip of the iceberg.

This new study is like sending a team of detectives out to knock on every door in the neighborhood to ask, "Did anyone in your house get sick, even if they never came to the clinic?" What they found was shocking.

The Hidden Epidemic

The researchers discovered that the actual number of people getting sick in the community was 4.2 times higher than what the clinic records showed.

Think of it like a forest fire. The health clinic only sees the trees that are burning right next to the road and are close enough to call for help. But this study walked deep into the woods and found that the fire was actually raging in the dense forest far away from the road, consuming many more trees than anyone realized. The "community attack rate" (the percentage of people getting sick) was 1.1%, which sounds small, but in a neighborhood of millions, that's a massive number of people.

The Tragedy of Waiting

The study also looked at why people were dying. They found a heartbreaking pattern: Time is the enemy.

If a family waited four or more days to take a sick person to the doctor, the risk of death skyrocketed. The researchers calculated that waiting this long made the chance of dying 32 times higher.

Imagine diphtheria as a fast-moving flood. If you see the water rising and immediately move to higher ground (go to the clinic), you are safe. But if you wait to see if the water will go down on its own, by the time you decide to move, the flood has already swept you away. Sadly, two out of every three people who died never made it to the hospital at all; they passed away at home, often because they waited too long or didn't have access to care.

The Shield That Wasn't There

Vaccination is like a shield against this disease. The study found that if you had the vaccine, your chances of surviving were much higher (about 57% more effective at preventing death).

However, the neighborhood's defenses were weak:

• The Routine Shield: Only about 56% of the children who should have been vaccinated by their regular doctors actually had the shield.
• The Emergency Shield: When the government tried to run a special "catch-up" campaign to vaccinate everyone who missed out, it mostly ended up vaccinating people who were already protected. It was like giving extra umbrellas to people who were already dry, while leaving the people in the rain without any. As a result, nearly 12% of the children who needed protection were still standing in the storm.

The Takeaway

This paper tells us that the official numbers of sick people were a massive underestimation. The real story is that the disease was spreading quietly in homes, and people were dying because they waited too long to get help and because too many children didn't have their protective shields.

The lesson for the future?

1. Look deeper: Don't just count the people walking into the clinic; go into the homes to find the hidden cases.
2. Act faster: Treat the disease like a fire that needs to be put out immediately, not something to watch and wait on.
3. Fix the gaps: Make sure the vaccination campaigns actually reach the people who are not already protected, rather than just re-vaccinating those who are safe.

In short, the neighborhood was in more danger than anyone knew, and the only way to stop the next wave is to find the hidden sick, act immediately, and make sure every child has a shield.

Technical Summary

Technical Summary: Community Burden of Diphtheria in Kano State, Nigeria (2023–24)

1. Problem Statement

Nigeria has been grappling with its largest recorded diphtheria outbreak since late 2022, with Kano State serving as the epicenter. While facility-based surveillance systems documented over 25,000 confirmed cases, these figures likely represent a significant underestimation of the true disease burden. Facility data often miss cases that do not seek medical care, fail to reach health facilities, or occur in remote areas. Consequently, critical gaps existed regarding the actual community attack rates, mortality figures (particularly outside hospitals), vaccination coverage levels, and the specific determinants driving infection and death within the general population.

2. Methodology

To address these surveillance gaps, the researchers conducted a retrospective, population-based household survey between September and October 2024.

• Sampling Design: The study utilized spatially randomized cluster sampling, selecting 65 clusters with approximately 15 households per cluster.
• Recall Period: Data collection covered the period from January 2023 up to the time of the interview.
• Data Analysis: The team employed survey-weighted analyses to account for the complex sampling design. They utilized multivariable logistic regression to identify risk factors and conducted sensitivity analyses to ensure the robustness of their findings.
• Scope: The survey enrolled 7,998 individuals across 1,068 households.

3. Key Contributions

This study provides the first rigorous, population-based estimation of the diphtheria burden in Kano State, moving beyond facility-dependent metrics. Its primary contributions include:

• Quantifying the "Hidden" Burden: Establishing a multiplier effect between facility-reported cases and the actual community prevalence.
• Mortality Dynamics: Revealing that the majority of deaths occurred outside the healthcare system, a critical insight often missed by standard surveillance.
• Vaccination Gap Analysis: Evaluating the efficacy of both routine Expanded Programme on Immunization (EPI) and reactive vaccination campaigns, specifically identifying the overlap and gaps in coverage.
• Risk Factor Identification: Isolating delayed care-seeking and low vaccination status as the primary drivers of mortality.

4. Key Results

• Attack Rate Discrepancy: The community attack rate was estimated at 1.1% (95% CI 0.7–1.4). This figure is 4.2 times higher (95% CI 2.7–5.3) than estimates derived solely from facility-based surveillance, indicating a massive undercount in official records.
• Mortality and Case Fatality Ratio (CFR):
  • The overall CFR was 8.8% (95% CI 1.9–15.6).
  • Mortality was significantly higher among children under five, with a CFR of 21.3%.
  • Two-thirds of all deaths occurred at home, underscoring the failure of the current health system to capture severe cases.
• Determinants of Mortality:
  • Delayed Care-Seeking: A delay of four or more days in seeking care was associated with a drastically increased risk of death (Risk Ratio: 32.6; 95% CI 2.4–450.0).
  • Vaccination Status: Vaccination was strongly protective, with a calculated Vaccine Effectiveness (VE) of 57% (95% CI 34–72%) against death.
• Vaccination Coverage:
  • Routine EPI coverage among campaign-eligible children was only 56.6%.
  • Reactive vaccination campaigns were inefficient in reaching new populations; there was a 99.7% overlap between campaign recipients and those previously vaccinated, leaving 11.6% of eligible children unvaccinated.

5. Significance and Implications

The findings highlight a critical disconnect between surveillance data and the reality on the ground. The study demonstrates that the true burden of diphtheria in Kano State is substantially higher than reported, with a high mortality rate driven by systemic failures rather than just pathogen virulence.

The results necessitate a paradigm shift in public health response:

1. Surveillance Reform: There is an urgent need for enhanced community-based surveillance to capture cases that never enter the formal health system.
2. Decentralized Care: To reduce the 32.6-fold increase in mortality associated with delayed care, health interventions must be decentralized to bring treatment closer to affected communities.
3. Targeted Immunization: The low efficiency of reactive campaigns suggests a need for better-targeted vaccination strategies that specifically reach the unvaccinated "hard-to-reach" populations rather than re-vaccinating those already protected.

In conclusion, controlling future outbreaks requires addressing the dual challenges of delayed care-seeking and low routine vaccination coverage through community-integrated health strategies.