Mexican President Andrés Manuel López Obrador announced on Twitter Sunday evening that he contracted the coronavirus.

Obrador said his symptoms are mild and he is undergoing medical treatment.

"As always, I'm optimistic. We'll all get by," the 67-year-old president tweeted.

Obrador had repeatedly refused to wear a mask and resisted calls for mask mandates and other restrictions despite the growing number of cases in the country.

-ABC News' Anne Laurent and Kirit Radia

There are 110,628 people currently hospitalized in the U.S., the fewest since Dec. 14, the COVID Tracking Project reported.

The tracking project said that hospitalizations are decreasing across the country.

"For the first time since November 3rd, no state has over 600 people per million hospitalized with COVID-19," the tracking project tweeted.

The New Zealand Health Ministry announced Sunday it recorded its first coronavirus case from within the community in three months.

New Zealand has been reporting positive cases in managed isolation, which requires anyone who travels into the country, both citizens and non-citizens, to go into a 14-day quarantine in a designated facility. There have been no cases from within the community since November.

The female patient traveled in Spain and the Netherlands late last year for work, according to officials.

"While in the Netherlands she was in contact with family members, who subsequently tested positive for COVID-19," the health ministry said in a statement.

The patient arrived in Auckland on Dec. 30 following a trip from the United Kingdom with a layover in Singapore, according to the Health Ministry. The patient stayed at a hotel during her 14-day quarantine and tested negative on Jan. 2 and Jan. 10, according to the Health Ministry.

"The person started developing mild symptoms on 15 January and these progressively got worse. The person had a test taken on Friday 22 January and self-isolated at home," the Health Ministry said in a statement.

The patient's roommate hasn't shown any symptoms so far, but has been tested and is isolating at home, according to the Health Ministry.

The Health Ministry said it is contact tracing and carrying out rapid genome sequencing to see if there's a match to one of the more transmissible variants.

-ABC News' Christine Theodorou

Chicago Public Schools announced Sunday they will postpone the in-person start for kindergarten through eighth grade teachers after the Chicago Teachers Union announced voted over the weekend to continue remote work.

Teachers were scheduled to report to in-person learning on Monday, but that was pushed back to Wednesday, according to CPS. "While we agree with our labor partners on many aspects of a smooth expansion of in-person learning, our discussions are ongoing. To ensure we reach a resolution without a disruption to student learning, we’ve agreed to push back the return of K-8 teachers, staff to Wed, 1/27," CPS said in a statement.

Kindergarten through eighth grade are slated to return to in-person learning on Feb. 1. Pre-K and special education students have been going to in-person classes since the beginning of the month.

In a statement, the union said that there was concern among members regarding the rise in COVID-19 cases and the small number of students who have opted to return to in-person learning.

Chicago Public Schools said 37% of parents surveyed intend for their kids to return, according to ABC station WLS. The union noted that the number of eligible students who can currently return to in-person is also low.

"But the fact of the matter remains this: 19% of students have returned. The district doesn’t need anywhere near all of our membership to return to meet that need," CTU said in a statement.

Some parents who wished to have their children return to class in person took up their pleas directly to the union this weekend.

"Parents should not be vilified or bullied for needing a choice to return to in-person learning," Sarah Sachen told WLS.

Before the end of the month, London will begin piloting COVID-19 vaccination sites that will be open 24 hours a day, seven days a week, according to Nadhim Zahawi, the United Kingdom's minister for COVID-19 vaccine deployment.

England's National Health Service (NHS) will trial the round-the-clock vaccination sites at hospitals in the British capital.

"And we'll look at how we expand that," Zahawi told Sky News in an interview Monday.

Zahawi said the key with 24-hour vaccination sites is to ensure vulnerable populations, such as people over 80 and others deemed high-risk, are still being targeted.

"At the moment, the challenge is obviously supply -- limited supply vaccine that needs to get into the arms of the most vulnerable four cohorts. So the targeting has to be really, really precise so that we can protect them by mid-February," he added.

"So if you just want to chase volume, chase speed and not accuracy, 24 hours works really well," he continued. "If you want to chase both accuracy, protecting the most vulnerable and of course speed, then you want to do what we're doing which is primary care networks, hospitals, large vaccination centers and of course pharmacies."

As of Saturday, 3,857,266 people in the U.K. have received the first of two doses of a COVID-19 vaccine, according to the latest data published on the U.K. government's website.

The U.K. Medicines and Healthcare products Regulatory Agency (MHRA) has approved three COVID-19 vaccines for use to date -- one developed by U.S. pharmaceutical giant Pfizer and its German partner BioNTech, a second developed by England's University of Oxford and British-Swedish pharmaceutical company AstraZeneca, and a third developed by American biotechnology firm Moderna.

The U.K. -- an island nation of 66 million people made up of England, Scotland, Wales and Northern Ireland -- has confirmed more than 3.3 million cases of COVID-19 since the start of the pandemic, including over 89,000 deaths. There were 38,598 new cases and 671 additional deaths confirmed over the past 24 hours, according to the latest data.