Merck and Co Inc (MRK)

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“When someone is infected by a virus, it makes the cells in their body produce copies of itself. This drug works by causing the machinery that reproduces Covid-19’s genetic material to make mistakes, thereby stopping effective replication. That mode of action could cause problems with our own cells, and while reports are that the drug is well tolerated, we still don’t have full details of any side effects. It’s worth noting that people involved in the trial were instructed to abstain from heterosexual sex or use contraception. While this is routine practice with some other medicines, such as cancer chemotherapy, it suggests that the drug has the potential to cause birth defects should someone become pregnant.

“This news is positive, but we must not be carried away. It would be good for the 50% of patients that it could save from getting seriously ill, but not so much for those who are still hospitalised, despite taking it. There’s currently no way of knowing which group someone will be in. It’s worth remembering that another drug, dexamethasone, was found to save around a third of people who would have died with severe Covid-19, but its clinical introduction did not prevent nearly 100,000 deaths in the UK in the past twelve months.