The mosquito-borne illness yellow fever has been found in further districts in Brazil, including the state of Sao Paulo where 20 cases have recently been identified. Now, almost all of Brazil is said to be at risk of the serious viral illness. Although the WHO have identified a handful of states where yellow fever has still not been found, in practice all travellers should be vaccinated before travelling as it is very likely that journeys to Brazil will now include yellow fever areas.
So far, only one person infected with yellow fever in Brazil has been shown to have subsequently carried it to their home country: this solitary case was diagnosed in the Netherlands two weeks ago. With mosquito borne illnesses, it is often a fear that epidemics of disease can be transmitted to other parts of the globe when returning infected patients are bitten by local mosquitos, which subsequently also become infected with yellow fever. Whilst there is no chance of this occurring in the chilly Netherlands, if the patient was a resident of a country in the tropics, for example, the situation may have been considerably different.
Although the case numbers in Brazil are fairly low, because of the seriousness of the infection, the number of fatalities is disproportionately high: although only 145 patients are suspected of having the disease, 20 have already died. Brazilian authorities have therefore instigated a mass vaccination campaign of over 21 million people. Apart from the problematic logistical considerations of such a large programme, there are further difficulties as supplies of yellow fever vaccine have been limited for some time.
In fact, a radical solution that is being utilised is for a much-reduced dose of the yellow fever immunisation to be trialled. One fifth of the usual immunisation (0.1mls) will be given to 16 million of the patients, the other individuals receiving the usual 0.5mls dose. There is good evidence from some research that this dose-sparing approach might well be very effective: 4 studies have now shown that the lower dose is able to provoke the same level of yellow fever antibodies in patients as the full dose.
Of course, measuring levels of antibodies in the two dosing groups is not the same as comparing the proportion of patients who are protected from the disease – technically speaking, the antibody levels are just a surrogate for the immunity achieved against the yellow fever disease. Simply put, we hope that the similar antibody levels will mean that all the patients will be equally protected against yellow fever. But we don’t know for sure. With 21 million vaccinated patients in Brazil, we may be able to address this point very soon.
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