# Impact of the COVID-19 pandemic and typhoid conjugate vaccine introduction on typhoid fever in Nepal

## TL;DR

Nepal's typhoid vaccine reduced cases in children but increased in older groups after pandemic restrictions eased.

## Contribution

Quantifies typhoid decline in vaccinated children and unexpected rise in older populations post-pandemic.

## Key findings

- Typhoid cases dropped sharply during the pandemic before vaccine introduction.
- Vaccine rollout reduced typhoid in eligible children but increased cases in older, unvaccinated groups.
- Relaxation of pandemic measures correlated with rising typhoid in vaccine-ineligible populations.

## Abstract

While typhoid conjugate vaccines (TCV) offer promise for reducing risk in endemic settings, their population-level impact remains unclear. In 2022, Nepal introduced TCV nationally on the heels of the COVID-19 pandemic, which disrupted healthcare services, surveillance, and potentially typhoid transmission dynamics, complicating vaccine impact evaluation. We investigated the impact of TCV introduction amid shifting typhoid burden during the pandemic.

We analyzed blood culture data from four Kathmandu Valley health facilities, comparing culture positivity for Salmonella Typhi across three periods: pre-pandemic (January 2018-March 2020); pandemic, pre-vaccine introduction (April 2020-March 2022); post-vaccine introduction (April 2022-April 2024). We used multivariable logistic regression to assess S. Typhi positivity, adjusting for month and site, stratified by TCV-eligible children and older, TCV-ineligible populations.

Between January 2018 and April 2024, 62,236 blood cultures were performed. S. Typhi blood culture positivity decreased from 2.11% pre-pandemic to 0.59% during the pandemic (p < 0.001) and remained low at 0.69% after TCV introduction. Among TCV-eligible children (15 months to 15 years), odds of S. Typhi positivity during the pandemic were 47% lower than the pre-COVID period (aOR 0.53, 95% CI 0.29-0.90) and continued to decrease by 75% post-TCV introduction (aOR 0.25, 95% CI 0.11-0.55). In contrast, among vaccine-ineligible individuals (≥16 years), odds of positivity during the pandemic were 77% lower than the pre-COVID period (aOR 0.23, 95% CI 0.16-0.31) but increased by 59% following TCV rollout (aOR 1.59, 95% CI 1.14-2.27). Sensitivity analyses restricted to pathogen-positive cultures yielded similar results.

S. Typhi blood culture positivity declined sharply during the pandemic before TCV introduction. The subsequent rollout of TCV substantially reduced typhoid burden in vaccine-eligible children; however, rising cases among older, vaccine-ineligible populations following the relaxation of pandemic measures highlights the need for additional control measures such as improved water and sanitation infrastructure and broader age eligibility for typhoid vaccination.

Typhoid fever is a serious illness caused by bacteria that spread through contaminated food and water. In 2022, Nepal introduced a new typhoid vaccine for children, just as the country was recovering from the COVID-19 pandemic. The pandemic transformed how people accessed healthcare and may have also affected how typhoid spread. We wanted to understand how both the pandemic and the vaccine rollout influenced typhoid infections over time. Using results from four hospitals in the Kathmandu Valley between 2018 and 2024, we compared the number of typhoid cases before the pandemic, during the pandemic before typhoid vaccine introduction, and after typhoid vaccine introduction. We found that typhoid cases dropped sharply during the pandemic, even before the typhoid vaccine introduced. After the vaccine was introduced, typhoid continued to decline in children who were eligible for the vaccine. However, cases increased in people who were not vaccinated. Our findings show that the vaccine helped protect the age group that received it, but highlights the need for additional interventions such as improvements to sanitation and clean water infrastructure, and expanding vaccine access to more age groups to control typhoid in the future.