People talk outside an NHS COVID-19 vaccination center in London, Britain, on March 4, 2021. Future vaccines that have been tweaked to deal with new coronavirus variants will be fast-tracked for authorization, Britain's medicines and health care regulator announced Thursday. (Xinhua/Han Yan)

A man talks with a staff member at an NHS COVID-19 vaccination center in London, Britain, on March 4, 2021. Future vaccines that have been tweaked to deal with new coronavirus variants will be fast-tracked for authorization, Britain's medicines and health care regulator announced Thursday. (Xinhua/Han Yan)

Photo taken on March 4, 2021 shows an NHS COVID-19 vaccination center in London, Britain. Future vaccines that have been tweaked to deal with new coronavirus variants will be fast-tracked for authorization, Britain's medicines and health care regulator announced Thursday. (Xinhua/Han Yan)

Photo taken on March 4, 2021 shows a sign for an NHS COVID-19 vaccination center in London, Britain. Future vaccines that have been tweaked to deal with new coronavirus variants will be fast-tracked for authorization, Britain's medicines and health care regulator announced Thursday. (Xinhua/Han Yan)

A man is seen at an NHS COVID-19 vaccination center in London, Britain, on March 4, 2021. Future vaccines that have been tweaked to deal with new coronavirus variants will be fast-tracked for authorization, Britain's medicines and health care regulator announced Thursday. (Xinhua/Han Yan)

People are seen at an NHS COVID-19 vaccination center in London, Britain, on March 4, 2021. Future vaccines that have been tweaked to deal with new coronavirus variants will be fast-tracked for authorization, Britain's medicines and health care regulator announced Thursday. (Xinhua/Han Yan)