Vaccines have “weakened” but not broken the link between Covid-19 infections and serious illness, Wales’ chief medical officer has said.

Dr Frank Atherton told a press conference in Cardiff that Wales was in the third wave of the pandemic, with “extensive community transmission” across the country.

The incidence rate in Wales has reached 136 cases of Covid-19 per 100,000 people – the lowest of the UK nations.

On Monday, Public Health Wales reported a further 1,190 cases, taking the total since the start of the pandemic to 225,691.

Dr Atherton said the reproductive number – or R rate – for Wales was estimated between 1.8 and 1.9, meaning each person with Covid-19 was going on to infect almost two more people.

Wales has the highest levels of vaccination in the UK, with 90% of adults having had one dose and more than 70% receiving both doses.

“It’s very easy to think that once we’ve been fully vaccinated, we’re somehow invincible,” Dr Atherton said.

“It’s very tempting to think that once we reach a certain level of vaccination coverage in Wales that the pandemic will be over, but unfortunately the virus is still with us, cases are still rising during this third wave.

“Although we have weakened the link between the virus and serious illness, we haven’t broken it completely.

“If cases come close to or even exceed the December peak, that’s a lot of people who could become ill, who would need to self-isolate and who would miss work, and possibly develop long Covid.”

He said it was important for people to maintain social distancing, regularly wash their hands, meet outdoors rather than indoors, work from home where possible and continue to wear face masks in indoor public spaces.

Dr Gill Richardson, deputy chief medical officer for vaccines, warned that only about 75% of those aged 18 to 39 had received their first jab.

“These young adults are vulnerable to infection, serious illness and life-changing long Covid as well as loss of earnings if they have to self-isolate as cases of the Delta variant are increasing in this age group,” she said.

“Our focus at the moment is on maximising protection, particularly amongst younger people.”

Dr Richardson said health boards were running walk-in Pfizer vaccination clinics for younger people and urged them to come forward.

She told the press conference Wales was not seeing the same level of hospital admissions as during the winter but these could be expected to rise.

In December, around 10% of people with Covid-19 in Wales were admitted to hospital but this has fallen to around 1% for cases of the Delta variant, Dr Richardson said.

First Minister Mark Drakeford is due to outline any changes to Covid-19 restrictions in Wales on Wednesday.