The PMâ€™s comments come as eight further cases of the variant have been confirmed in England, bringing the total number in the country to 13. Cases of Omicron have now reached 22 across the UK.
Earlier in the day, Dr Jenny Harries, chief executive of the UK Health Security Agency, urged people not to socialise if they do not need to in the run-up to Christmas, and Johnson was asked at a Downing Street press conference if festive parties should be cancelled.
He replied: â€œSo the answer is no. The guidance remains the same, and weâ€™re trying to take a balanced and proportionate approach.
â€œWeâ€™ve got the measures in place to fight Delta, which we think are appropriate, and then weâ€™re bringing in some tougher measures to stop the rapid seeding of Omicron in this country to give us the time we need to get the boosters in and of course to find out more.â€
Asked whether parties and Nativity plays should be scrapped, Johnson said: â€œWe donâ€™t want people to cancel such events and we think that, overwhelmingly, the best thing for kids is to be at school, as I have said many times throughout this pandemic.
â€œWhat we are doing is trying to take a balanced and proportionate approach to the particular risk that seems to be posed by Omicron â€“ certainly is posed by Omicron â€“ focused in particular on measures at the border.â€
He described the new measures as â€œthe right balanceâ€. The UKâ€™s response has included stricter travel measures, insisting on increased mask use and expanding the booster vaccination programme.
It is the booster campaign which the government believes provides the greatest line of defence against Omicron and Johnson used a Downing Street press conference to promise another â€œgreat British vaccination effortâ€ to deliver millions of jabs.
â€œWeâ€™re going to be throwing everything at it, in order to ensure that everyone eligible is offered that booster in just over two months,â€ he said.
Health secretary Sajid Javid said the booster programme would be put â€œon steroidsâ€ to meet the target, while NHS Englandâ€™s chief executive Amanda Pritchard said staff are working at â€œbreakneck speedâ€.
Harriesâ€™ comments caused alarm in the hospitality industry and helped fuel a Tory backbench revolt which saw Conservatives rebel over restrictions which came into force in England on Tuesday morning to tackle Omicron.
She told BBC Radio 4â€™s Today programme that even if our â€œvaccines appear to be effective, but we find that the variant is more highly transmissible, having lowish grade infection, but in very large numbers of the population, (it) could still be a significant impact on our hospitals.
â€œAnd, of course, our behaviours in winter and particularly around Christmas, we tend to socialise more, so I think all of those will need to be taken into account.â€
Asked whether people should be told to work from home in England, as is happening in Scotland, Wales and Northern Ireland, she said: â€œWeâ€™ve seen that not everybody has gone back to work and Iâ€™d like to think of it more in a general way, which is if we all decrease our social contacts a little bit, actually that helps to keep the variant at bay.â€
She suggested â€œbeing careful, not socialising when we donâ€™t particularly need toâ€ and getting a booster jab.
Kate Nicholls, chief executive of the UKHospitality trade body, said this â€œchilling talkâ€ could hammer the sector ahead of its busiest period.
She said bookings have already been â€œcancelled and plans changedâ€ by some customers.
Johnson rejected suggestions that advice on working from home should be reinstated in response to the concerns about Omicron, which scientists believe could be more transmissible than the dominant Delta variant and may render vaccines less effective because of the extent of its mutations.
Asked why that advice was not in place in England, Johnson said: â€œThis is not something that we think currently is necessary.â€