Johnson has previously warned that the swift spread of the B.1.617.2 variant of concern first identified in India could derail his plans to end England's COVID restrictions on June 21.
"I can see nothing in the data at the moment that means we can't go ahead with step four, or the opening up on June 21st, but we've got to be so cautious."
The number of doctors -- 2.8 per 1,000 people -- is also "well below EU averages", while Britain has the second-lowest number of hospital beds in Europe.
The event will discuss how to tackle misinformation about vaccines, including the role social media companies can play in stopping the spread of damaging falsehoods.
"We must work collectively to ensure people across the world can reclaim their lives and livelihoods."
One of the last opinion surveys before Thursday gave Johnson's ruling party a double-digit lead in the seat, which if confirmed would represent a serious blow to Labour leader Keir Starmer ahead of the next general election due in 2024.
Johnson's government has set out a roadmap to end lockdown restrictions in stages as widespread vaccinations help to suppress infections.
Asked about that possibility during a campaign visit in northern England, Johnson said: "I think we've got a good chance, a good chance, of being able to dispense with one-metre plus."
The Daily Mail newspaper said Johnson said at an October meeting at Downing Street: "No more ****ing lockdowns - let the bodies pile high in their thousands."