Some 17% of new coronavirus cases in the last two weeks were in fully-vaccinated people.
On Monday evening, 1,522 new cases of the virus were announced by officials at the Department of Health.
Over 200 people are in hospital with the virus, with 34 suffering in ICU.
However, Chief Medical Officer Dr Tony Holohan noted that the upturn was what NPHET expected.
He said: "Over the past fortnight we have reported almost 20,000 cases. While 17% of these cases were in people who are doubly vaccinated, this is entirely in keeping with what we expect as an increasing proportion of our population get vaccinated. It's important to remember that this does not mean vaccines are not effective.
"While they will not prevent every case, they provide excellent protection against severe disease and significantly reduce the risk of hospitalisation.
"If you are awaiting or have recently received your second vaccine dose, please be aware you are not yet fully vaccinated.
"You will be fully vaccinated one week after your second dose of Pfizer and two weeks after your second dose of Moderna and AstraZeneca. If you have received Janssen, you should not regard yourself as fully vaccinated until two weeks after."
Dr Ronan Glynn, Deputy Chief Medical Officer, Department of Health added: "It is important to continue to follow the public health advice. Please remember that you should stay at home if you have any cold or flu symptoms even if you are fully vaccinated, because you could still transmit COVID-19.
"If you wait to isolate until you get the results of your test, you will be much more likely to pass it on to others in your family and community. We must work together to continue to limit the spread of the disease in our communities and key to this is not attending the workplace or socialising if you have any symptoms.”
