Novel model suggests benefit of vaccination against Salmonella in sub-Saharan Africa

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Researchers have created a model to predict the impact of vaccination against invasive non-typoidal Salmonella (NTS) in Bamako, Mali, demonstrating that introduction of a pediatric vaccine could markedly reduce case numbers and mortality rates in children.

In sub-Saharan Africa invasive strains of NTS have been reported to cause systemic bacteremia, and severe infections could fatal in vulnerable groups, such as infants under the age of 5.  A surveillance system, which monitored bacterial invasive disease in pediatric patients, highlighted that with vaccines against Haemophilus influenzae type B and Streptococcus pneumonia there is now a decreasing number of cases; making NTS the predominant invasive pathogen.

Although the relative threat of NTS has increased, little is currently known about the disease reservoir or transmission; without this vaccination is the most promising disease-control strategy. Vaccines against NTS are now approaching clinical trials; therefore the researchers hoped to model the potential impacts of different vaccine strategies on hospitalization and mortality rates in Bamako, Mali.

The study, published recently in PLOS Neglected Tropical Diseases, collected data on 515 patients under 5 years old with confirmed invasive NTS in Bamako hospital. The team analyzed this data in conjunction with information on the efficacy and coverage of other vaccines and general demographic data to model the public health impact of an NTS vaccination regimen.

The group, led by Myron Levine (University of Maryland; MD, USA), demonstrated that distribution of a vaccine against S. typhimurium and S. enteritidis to newborns in a 3-dose schedule could reduce invasive NTS cases by up to 73%, and avert up to 43% of deaths. Moreover, the researchers discovered that if a catch-up vaccination campaign, which would target older children, was simultaneously launched this could further improve vaccine impact.

This study does have some limitations; information on invasive NTS was only available from hospitalized children, which may exclude milder cases, or very severe cases unable to reach hospital. However, the findings could be a useful public health tool with impacts not only in Mali but also in other countries with a high NTS burden.

In the article the researchers concluded: “Since neither the reservoir of infection nor the modes of transmission of NTS to young children have heretofore been identified, vaccination currently represents the most plausible interventional strategy for reducing the burden of iNTS disease,”

“Even at early stages in development of the candidate vaccines to prevent iNTS disease, a mathematical model of what the vaccine might achieve at the future public health level becomes a useful, hopefully predictive tool.”

Sources: Bornstein K, Hungerford L, Hartley D et al. Modeling the Potential for Vaccination to Diminish the Burden of Invasive Non-typhoidal Salmonella Disease in Young Children in Mali, West Africa, PLoS Negl. Trop. Dis. doi:10.1371/journal.pntd.0005283 (2017) (Epub ahead of print); www.eurekalert.org/pub_releases/2017-02/p-rmi020217.php

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