The frontrunners in the COVID-19 vaccine race have emerged with different success rates for their shots in clinical trials, but what does that mean for the global fight against the pandemic?
US drugmakers Pfizer-BioNTech and Moderna have each said that their coronavirus vaccines have an efficacy rate of around 95% and a Russian project touted 92% efficacy for its Sputnik V vaccine.
Britain's AstraZeneca has announced an average efficacy rate of 70%, still well above the 50% rate that US regulators have said they want to see before approving a COVID-19 vaccine for use.
What are efficacy numbers?
If a vaccine has an efficacy of, say, 80%, it means that if 100 people who have not previously been infected by the coronavirus are given the vaccine, on average 80 of them will not get the disease that the virus causes: COVID-19. These rates relate to vaccines administered and monitored in controlled circumstances, such as clinical trials.
Does that mean an individual's protection level is the same?
No. A person immunised with a vaccine that has, say, 80% efficacy is very likely to be protected from getting the disease with symptoms, especially severe ones.
They are also very likely to be protected from asymptomatic disease - but this, depending on the vaccine, may be less certain.
Even with 95% efficacy, there is no absolute guarantee of protection for any particular individual.
What does this mean in the real world?
There is a difference between efficacy rates obtained in clinical trials and effectiveness - the real-world protection rate of a vaccine when it is rolled out.
"Efficacy says: 'Does it work?' Effectiveness says: 'Can it be applied? Can you carry the efficacy to the people?'," said Marcel Tanner, an epidemiologist and president of Switzerland's Academies of Arts and Sciences.
In the real world, a vaccine's effectiveness can be influenced by multiple, unpredictable factors including, for example, the rate of spread of a virus; how many, or few, people adhere to the optimum dosing schedule and timetable; how individuals' immune systems respond; whether the vaccine was stored at the correct temperature; whether people know, or don't know, if they've been exposed before.
Generally, a vaccine's real-world effectiveness tends to be slightly lower than its efficacy.
Will these vaccines stop the COVID-19 pandemic?
Experts say this is unlikely. More realistic, they say, is that we will have to live alongside the SARS-CoV-2 virus.
Evidence so far suggests that COVID-19 vaccines developed by Pfizer-BioNtech, Moderna and AstraZeneca will help stop people developing the disease. Only AstraZeneca's data, so far, shows signs that its shot may also help prevent transmission of the virus.
"Protection against illness has value for an individual," said Penny Ward, a visiting professor in pharmaceutical medicine at King's College London. She added, however, that vaccines that do not prevent transmission will not halt the pandemic.
"Until the vaccination and other measures result in the virus being close to elimination in any particular country, and worldwide, there will still be a need for distancing, masking and hand washing to reduce transmission further than will be achieved by the vaccine alone," said Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene and Tropical Medicine.
"(A) vaccine is no good until people are vaccinated, and even then, it will not result in a situation where all other protective measures can be immediately abandoned."
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