At least seven fully-vaccinated students at Stanford University have tested positive for coronavirus.
Officials at the Ivy League school in California's Bay Area reported the breakthrough infections in a letter to students on Thursday.
They said all of the cases were confirmed in the last week and all seven students were symptomatic.
'As you have seen in the national news, cases of COVID-19 have been ticking upward,' officials wrote in the letter.
'We are seeing some of this in our own community, where we are experiencing an increase in the number of student COVID cases, including among fully vaccinated individuals.'
Stanford is among nearly 600 universities and colleges nationwide that have required students and faculty to be vaccinated against COVID-19 before coming back to campus this fall.
The school is enforcing its mandate by requiring proof of vaccination status and says it will offer exemptions for religious or medical reasons on a case-by-case basis.
At least seven fully-vaccinated students at Stanford University have tested positive for coronavirus, school officials announced Thursday. Pictured: Students socially distance during a class held outside at the Ivy League in California
The likelihood of contracting coronavirus after being fully vaccinated is very low, with so-called 'breakthrough' cases representing only a small percentage of new cases reported around the US.
Experts have sought to quell concerns over such cases by reminding that COVID-19 vaccines are still highly effective in preventing severe symptoms, hospitalization and death.
Less than three percent of people hospitalized with COVID-19 are vaccinated, according to the CDC, and only 1,063 deaths have been reported among vaccinated people.
However there are fears that vaccines may not be as effective at battling new more contagious strains of the virus, including the Delta variant wreaking havoc nationwide.
At Stanford, officials have stressed the importance of vaccination while continuing to require face coverings across all open spaces on its 8,180-acre campus.
'Vaccinations are effective at preventing severe illness. However, we need everyone to be vigilant,' officials wrote in Thursday's letter. 'The variants remain of concern.'
According to the school's dashboard, some 8,300 students are fully vaccinated and 330 are partially vaccinated.
The school recently loosened its coronavirus testing requirements, allowing for vaccinated students and staff to stop submitting to weekly
