New Italian law obliges children to be properlt vaccinated to be able to attend school, BBC reports.
Children have to prove that they have all the required vaccines, the law stipulates.
Parents risk being fined up to €500 (£425; $560) if they send their unvaccinated children to school. Children under six can be turned away.
The new law came amid a surge in measles cases. The deadline for certification was due to be 10 March after a previous delay - but as it fell on a weekend, it was extended to Monday. It follows months of national debate over compulsory vaccination.
Under Italy's so-called Lorenzin law - named after the former health minister who introduced it - children must receive a range of mandatory immunisations before attending school.
They include vaccinations for chickenpox, polio, measles, mumps, and rubella.
Children up to the age of six years will be excluded from nursery and kindergarten without proof of vaccination under the new rules.
Those aged between six and 16 cannot be banned from attending school, but their parents face fines if they do not complete the mandatory course of immunisations.
Now everyone has had time to catch up," Health Minister Giulia Grillo told La Repubblica newspaper.