Stanford University reported at least seven confirmed cases of COVID-19 among fully vaccinated students this week.

All seven students were symptomatic, according to the school.

“As you have seen in the national news, cases of COVID-19 have been ticking upward,” Stanford University officials said in a letter to students on Thursday. “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.”

Although the chances of contracting the novel coronavirus after being fully vaccinated are very rare, so-called breakthrough cases are still possible. Although COVID-19 vaccines have shown to be highly effective, they do not block the virus 100% of the time, meaning that some breakthrough infections occur after vaccination. This is normal and expected, and it’s not proof that the vaccines aren’t working, experts said.

However, health officials and experts alike have warned that new, more contagious variants of the novel coronavirus may be more effective at evading vaccines.

Stanford University officials said “vaccination continues to be our best defense against severe COVID-19 infection.” Face masks, physical distancing and regular hand washing “remain powerful ways to prevent the transmission of COVID-19,” officials added.

All students, faculty, staff, contractors and visitors are required to wear face coverings in spaces that are open to the public on the private university’s 8,180-acre campus in Stanford, California, about 20 miles northwest of San Jose. Face coverings in crowded indoor spaces on campus are recommended.

Stanford University recently loosened its COVID-19 testing requirements for fully vaccinated students who are living on campus or are coming to campus frequently this summer. Those individuals are no longer required to test weekly for COVID-19 or complete daily submissions on the school’s “Health Check” tool if they are verified as being fully vaccinated against the disease.

Overall, Stanford University has reported at least 257 confirmed cases of COVID-19 since the start of the pandemic. Some 8,300 students are fully vaccinated against the disease, while 330 are partially vaccinated, according to data posted on the school’s website.

Stanford University’s Vaden Health Services is connecting with each student who tests positive for COVID-19 to identify and notify their close contacts, who will be encouraged to get tested as soon as possible. Students who test positive will be provided with medical care and isolation space, according to the school.

“Again, vaccinations are effective at preventing severe illness. However, we need everyone to be vigilant,” Stanford University officials warned in the letter Thursday. “The variants remain of concern.”