The writer, a Nobel laureate in physiology or medicine, chairs the WHO Science Council
The rapid spread of the Delta variant of Sars-Cov-2 has dashed hopes that the pandemic might soon be over. It has also raised the prospect that yet more dangerous variants, including those impervious to vaccines or prior infections, might arise.
While we lack complete knowledge of how the most dangerous variants of Sars-Cov-2 are generated, we do know that these emerge through mutations to the genome that occur when the virus replicates. Such mutations are more likely to arise in immune-compromised individuals in whom the virus can replicate repeatedly for extended periods. Variants of special concern are those that infect cells more efficiently or evade immune responses.