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Pharmaceutical companies Pfizer and BioNTech have announced their COVID-19 vaccine is effective in preventing the virus in 90% of participants. 

This number is huge -- flu vaccines are only 40% to 60% effective -- and the news sent stocks soaring. 

But experts have warned the data has not yet been peer-reviewed, stressing cautious optimism is needed and maintaining social measures are important to prevent the virus from spreading.