The Chief Medical Officer has advised parents to exercise caution in relation to after-school activities as children return to classrooms across the country this week.

Dr Tony Holohan said: "As many children return to classrooms across the country today, it is important to highlight to both children and their parents that considerable preparations have been made across the country to reopen schools safely.

"Our data clearly shows that, to date, the school environment was not a major source of disease transmission.

"In order to go back to school as safely as possible, my advice to children is to wash your hands or use hand sanitiser regularly and follow the protocols operating in your school.

"Parents, please keep in mind the symptoms to look out for and do not send your child to school if they display any of them, instead help them to isolate and contact your GP for advice and arrange a test if appropriate.

"I would also urge caution when participating in activities with classmates outside the school environment, which have proven to be settings where outbreaks and transmission in children have occurred in the past.

"This will help to prevent further transmission and potential outbreaks."

His warning comes as a Professor of Immunology at Dublin City University said that we should prioritise the reopening of schools over other sectors of society.

Christine Loscher told RTÉ's Drivetime that Ireland is "racing too fast" in terms of reopening society.

She said schools should be prioritised, the impact of them reopening over a few weeks should be assessed and then return to a full capacity on public transport.

"We are opening too many things, too quickly. We have staggered things until now and it has worked well," she added.

Prof Loscher said procuring C02 monitors for every single classroom in the country should be "an absolute priority" and that she was "a little bit shocked" as to why there was not one in every classroom, when they are known to work.

She said schools have done everything they can to address social distancing but "we can do better with the ventilation".

Earlier, the Chair of the Irish Epidemiological Modelling Advisory Group said schools are not major sites of transmission of Covid-19, where effective mitigation measures are in place.

Professor Philip Nolan said on Twitter that this is affirmed by the experience of public health doctors who supported the safe opening of schools last March.

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"It is true that we face a new challenge: we have no experience of opening schools with the Delta variant and such high levels of circulating virus in young adults, adolescents and children," he said.

He said the Delta variant does not transmit differently, but it does transmit more efficiently, adding that "it is reasonable to assume that mitigation measures that were effective against Alpha will also be effective against Delta, though they need to be strictly observed, and the situation monitored".

Prof Nolan said that where in-school transmission has been observed, it tends to be among friendship groups and in high-contact situations.

This suggested that direct contact, droplets and short-range aerosols are the dominant modes of transmission in this setting.

He said international reports of increasing incidence in children and adolescents in Delta outbreaks does not necessarily imply that children are more susceptible to the Delta variant than the Alpha variant, or that schools are important sites of transmission.

He added that school openings and closures to date in Ireland have had minimal effect on incidence in the whole population and in children and adolescents of school-going age.

The Department of Health today reported 1,293 new cases of Covid-19.