Effectiveness of the TYPHIBEV® (Vi-CRM197 conjugate) vaccine introduction in Nepal: a test-negative, case-control study

Objectives We aimed to evaluate the effectiveness of TYPHIBEV®, which was
introduced through a catch-up campaign and routine immunization in Nepal,
in preventing blood culture-confirmed typhoid fever among children.
Methods We conducted a test-negative, case-control study where typhoid
test-positive cases were defined as vaccine-eligible pediatric patients
who tested positive for Salmonella Typhi by blood culture at participating
health facilities and test-negative controls were vaccine-eligible
patients who tested negative for S. Typhi on blood cultures. We matched by
age, location, date of blood culture, and surveillance site. We used
conditional logistic regression to calculate odds ratios(OR), and vaccine
effectiveness was calculated as 1-OR. Results Between October 1, 2022 and
December 31, 2024, 40 typhoid cases and 113 matched, test-negative
controls were enrolled. Both cases and controls were similar in
sociodemographic characteristics and water, sanitation and hygiene-related
living conditions. Among 39 cases and 108 controls with known vaccine
status, 20 cases(51%) and 91 controls(84%) had received TCV. Vaccine
effectiveness was 89% (95% CI: 65-97%) and was lower among children
<5 years (72%, 95% CI: -203-97%) compared with those 5-15 years
(98%, 95% CI: 80-100%). Vaccine effectiveness estimates did not
significantly differ when restricted to participants with documented
vaccination status. Conclusions The findings indicate that TYPHIBEV® was
highly effective in preventing typhoid fever up to 30 months following a
national introduction, with effectiveness estimates comparable to those
observed for Typbar-TCV®.