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Harvard increased the frequency of coronavirus testing for affiliates living in undergraduate housing Thursday, citing a surge of new cases less than two weeks since students returned to campus and on the third day of classes.

In the last week, Harvard University Health Services counted 94 positive cases, HUHS Director Giang T. Nguyen wrote in an email to affiliates late Thursday.

As a result, Harvard will require affiliates living in undergraduate housing to test three times a week — an increase from once a week.

“Many campuses across the nation are now facing high numbers of COVID-19 infections, so I am writing to encourage that we all act with increased caution in the days and weeks ahead,” Nguyen warned. “This pandemic is not over.”

The new cases bring the campus positivity rate to nearly 0.3 percent, according to the University’s COVID-19 dashboard. As of Thursday, there are more than 100 individuals in isolation and an additional 29 in quarantine.

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In his email, Nguyen touted high vaccination rates across campus — 95 percent of employees and 93 percent of students are fully vaccinated — but noted that the spread of the Delta variant makes it vital to maintain “all of our safeguards.”

“If you need to interact with many people in a single day, keep your mask on, limit each interaction to under 15 minutes, and don’t stand closer than necessary,” Nguyen wrote, advising that students keep the number of people closer than 6 feet from them “as low as possible.”

Last Friday, Cambridge announced it would reinstate its indoor mask mandate, effective Friday. The same day, Boston began to require face coverings in all indoor public settings.

Citing both orders, Nguyen recommended affiliates follow Harvard’s indoor mask requirement “on and off campus.”

Harvard also extended its indoor mask mandate on Thursday to include all strength and conditioning facilities, according to a Wednesday email Harvard Athletics sent student-athletes.

“With positive cases steadily increasing since all students moved onto campus and the volume of student-athletes that will be training in contained spaces for strength and conditioning sessions, we made the decision to require masks to make the activity as safe as possible,” Athletics Director Erin McDermott wrote in an emailed statement.

The requirement is a reversal of a policy implemented last week that required vaccinated student athletes to test twice weekly, which McDermott said would allow them to safely train and compete maskless. Athletes living in undergraduate dorms will also increase testing to at least three times per week.

In a separate email with Nguyen on Thursday, Dean of Students Katherine G. O’Dair called the rise in new infections “a turning point.”

“The Delta variant and our full-capacity residential campus has changed the landscape compared to early summer, and we need to respond as a community to the changing risk profile,” they wrote.

O’Dair and Nguyen added that the number of undergraduates in isolation housing — 63 as of Thursday — is only expected “to grow.”

“Let us be clear – this uptick is happening because this variant is highly transmissible, even within a vaccinated population. No one is to blame but the COVID virus itself, and it’s up to us to respond and protect each other,” O’Dair and Nguyen said.

Any undergraduates living off-campus are advised, but not required, to test three times a week, they said.

Ahead of the Labor Day long weekend, O’Dair and Nguyen asked students to modify any plans involving indoor activities and risky behaviors, such as partying or gathering with food and alcohol.

“Socializing with consistent and universal masking and minimizing the total number of close interactions you have in any 2-day window will make a big difference,” they wrote.

— Staff writer Benjamin L. Fu contributed reporting.

—Staff writer Christine Mui can be reached at christine.mui@thecrimson.com. Follow her on Twitter @MuiChristine.