According to a new study published in PLOS Medicine, mass vaccination campaigns in Africa have significantly reduced the number of yellow fever cases and deaths across the country since 2006.

Yellow fever is a viral disease found in tropical and subtropical areas of South America and Africa. The virus is transmitted to humans through the bite of an infected mosquito, usually the Aedes and Haemogogus mosquitos.

Many people who contract yellow fever do not experience symptoms. Those who have mild symptoms may experience sudden onset of fever, chills, severe headaches, general body aches, back pain, nausea, vomiting and fatigue.

Severe cases of the disease can cause symptoms such as heart dysfunction, brain dysfunction and liver and kidney failure, with liver failure leading to jaundice – a yellowing of the skin and whites of the eyes. Severe yellow fever symptoms can cause death in around 50% of cases.

There is no specific treatment for yellow fever. However, there is an effective vaccine available that prevents against disease transmission.

According to the research team of this most recent study, led by Neil Ferguson of Imperial College London in the UK, the last few years have seen an increase in funding for mass vaccination campaigns in the areas of Africa most affected by yellow fever.

But how are such campaigns impacting disease burden in these areas? According to the investigators, such figures are lacking. They therefore set out to gain more accurate numbers.

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Researchers estimate that mass vaccination campaigns in Africa have reduced the number of yellow fever cases and deaths by 27%.

To do so, they applied a “generalized linear regression model” to areas of Africa where yellow fever was reported between 1987 and 2011. The team used data on outbreaks of yellow fever, as well as environmental data and information from a serological survey, to estimate disease burden in these locations.

The researchers estimate that each year, between 51,0000 and 130,000 individuals are affected by severe yellow fever, causing between 19,000 and 180,000 deaths.

To determine the effectiveness of mass vaccination campaigns, the team took these figures and compared them with a “hypothetical scenario” that excluded the campaigns.

They estimate that since 2006, mass vaccination campaigns have reduced the number of yellow fever cases and deaths across Africa by 27%. In addition, they found that specific countries targeted by such campaigns may have experienced up to an 82% reduction in disease burden.

Commenting on their findings, the researchers say their model may be a useful tool in monitoring the effectiveness of vaccination campaigns in the future, particularly since their estimates are similar to previous global estimates after analysis of all currently available data.

“With the estimation method presented here, spatial estimates of transmission intensity can be combined with vaccination coverage levels to evaluate the impact of past or proposed vaccination campaigns, thereby helping to allocate resources efficiently for yellow fever control,” they add.

The team also notes that such information is important in future prevention of yellow fever, adding:

The impact of both past and future mass vaccination campaigns will prevent a substantial proportion of yellow fever disease burden for years to come.

[…]The achievements of the current mass vaccination campaigns could be sustained if a high level of immunization is achieved through a strong Expanded Program on Immunization (infant immunization) and preventive vaccination of populations that remain at risk, such as migrants or populations from as yet unvaccinated districts.”

Speaking of the limitations in their study, they note there is a “high level of uncertainty” in their estimates due to the lack of data available to them.

Furthermore, they point out that yellow fever is difficult to diagnose, as symptoms may be mistaken for other conditions. Therefore, the data used in this study may not be an accurate reflection of the number of yellow fever cases that occur in Africa.