More than four in ten youngsters in the 12-to-15 year age group have not had even one jab and number of those coming forward to get one has slowed.
Government figures revealed 56.7 percent in the age group, 23,675, had received at least one vaccine dose by February 13 — leaving 18,000 still unjabbed since the rollout began in September.
An extra 1,300 of those, just over three percent, came forward within the last month.
Healthy children are deemed to be at very low risk of developing severe symptoms through Covid. But the NHS says infection rates in schools remain high despite new cases falling overall and medics are concerned about infected youngsters passing the virus on to others more vulnerable.
Northamptonshire's vaccination programme director, Chris Pallot, admitted: “Many children aged 12 to 15 remain unprotected from Covid-19.
“By getting vaccinated children and young people can significantly reduce the risk of passing on the infection to others and it means they can continue to socialise safely at school and with those they care about.
“The message is loud and clear to anyone over the age of 12: Come forward for your Covid-19 vaccines – whether that’s your first, second dose or booster if you’re over 16.
"Delaying could potentially be dangerous, not only for you but for those you come in to contact with."
Nearly 6,000, around 30 percent, in the county's 16-and-17 year age group are also unjabbed.
Rules over wearing face coverings in classrooms were ditched in January and Prime Minister Boris Johnson last week announced he wants to end all remaining Covid restrictions a month earlier than planned.
That means from next week those with the virus will no longer need to isolate at home.
Mr Pallot hopes many will use the week off to get jabbed before the last restrictions are removed.
The county's mobile vaccine clinic is set up at Corby's Adrenaline Alley skate park this week in a bid to attract more youngsters.
Dozens more sites are open across the county for appointments and drop-ins — or both – including locations in Northampton, Corby, Kettering, Rushden, Wellingborugh, Thrapston, Towcester and Brackley.