CAPE TOWN (Reuters) - The idea of testing the BCG vaccine in Africa against COVID-19 was bound to cause controversy, yet officials say efforts underway could, if successful, give the continent a cheap and easily deployable weapon against the virus.

South African biotech group TASK this month started the continent's first trial to see whether 250 high-risk workers given a booster dose of BCG vaccine show better protection against COVID-19 than the same number given a placebo.

Three thousand health workers are eventually expected to be involved in the trial overall. Researchers expect preliminary results within six months in the year-long trial.

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Tests to see if the century-old Bacille Calmette-Guerin (BCG) vaccine, widely used in Africa for tuberculosis, protects against the new coronavirus have also begun in the Netherlands and Australia.

But unlike those countries, where BCG vaccinations at birth aren't the norm, South Africa's trial involves injecting people for a second time. BCG injections are mandatory at birth in South Africa, where TB is a leading killer.

"We are trying to see if ... reminding the immune system of that vaccine will make a difference," said Andreas Diacon, a professor at Stellenbosch University's medical school who founded TASK in 2005 to hunt for novelty medicines and vaccines.

"Perhaps it works, perhaps it doesn't, but it's definitely worth trying ... (because) we just need to know if it can help us through this COVID-19 crisis," he told Reuters at TASK's high-security mycobacteriology laboratory.

Trying out a new medical intervention in Africa always rings alarm bells because of an unfortunate history of big pharmaceutical companies using Africans as guinea pigs.

So when a French doctor last month suggested that the BCG should also be tested in Africa as a possible shield against COVID-19, it provoked a predictable social media backlash.

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Yet the vaccine is widely regarded as safe, and officials involved in the trial at Tygerberg hospital in Cape Town point out that it is so readily available in Africa and the developing world that poor countries with limited health resources could benefit massively from the discovery that the jab is effective.