Extra Covid tests are taking place in slotxo areas where new variants of coronavirus have been found.
This is known as surge testing - its purpose is to stop the spread of new forms of Covid at the very earliest stage.
How does surge testing work?
Increased testing and enhanced contact tracing is carried out in very specific locations - local authority areas, postcodes or even individual streets.
Some of this is carried out by health officials going door-to-door. It includes testing of people who do not have Covid symptoms.
The testing also helps scientists better understand the new variants.
Where is it taking place?
Surge testing is often used where there are outbreaks of new, more contagious variants.
Health Secretary Matt Hancock has said that there are now 86 local authorities with five or more confirmed cases of the Indian Covid variant, and surge testing is taking place in many of the areas where cases are highest.
Second jabs may be brought forward in the worst-affected areas.
What are the new variants?
Scientists have identified a number of "variants of concern" - these are newly spotted versions of Covid, which could be more contagious.
Also, because they're newer, less is known about how they respond to current vaccines.
The Indian variant is the latest to cause concern - others include the Brazilian, South African and UK (Kent) variants. They are generally named after the place where they were first seen.