Britain, which has one of the world's highest COVID death tolls, is one of the first countries to roll out its vaccination program, with some in Johnson's administration hoping a successful campaign will restore faith in his leadership.
Johnson said earlier this month that the government hoped to complete the vaccination program for care home residents and workers by the end of January to try to stop the spread of coronavirus which earlier in the pandemic ripped through such homes.
In a statement, England's National Health Service said it had offered a COVID shot to people living at more than 10,000 care homes with older residents while a small remainder have had their visits deferred during a local outbreak.
Johnson said it was "a crucial milestone in our ongoing race to vaccinate the most vulnerable against this deadly disease."
"There will be difficult moments to come, and the number of cases and people in hospital remains dangerously high. But vaccines are our route out of the pandemic, and having protected 8.9 million people with a first dose so far, our roll out program will only accelerate from here on."The government has promised to immunize the most vulnerable people against COVID-19 by mid-February and to offer a shot to every adult by autumn. It was so far given a first dose to 8.98 million people.