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.

Twenty-three reports of suspected deaths associated with COVID-19 vaccines have been submitted to Norway's national health registry as of Jan. 14, according to an updated statement from the country's drug regulator on Monday.

The Norwegian Medicines Agency said it "cannot rule out that adverse reactions to the vaccine occurring within the first days following vaccination (such as fever and nausea) may contribute to more serious course and fatal outcome in patients with severe underlying disease."

Thirteen of those reports have been assessed by the drug regulator, as well as the Norwegian Institute of Public Health.

"The reports suggest that common adverse reactions to mRNA vaccines, such as fever and nausea, may have contributed to a fatal outcome in some frail patients," Sigurd Hortemo, chief physician at the Norwegian Medicines Agency, said in the statement.

Steinar Madsen, medical director of the Norwegian Medicines Agency, told the state-run broadcaster NRK: "We are not alarmed by this."

"It is quite clear that these vaccines have very little risk, with a small exception for the frailest patients," Madsen said. "Doctors must now carefully consider who should be vaccinated. Those who are very frail and at the very end of life can be vaccinated after an individual assessment."

Norway is currently administering COVID-19 vaccines to the elderly and people in nursing homes with serious underlying diseases -- at first with just the Pfizer/BioNTech vaccine up until Jan. 15, and then also with the Moderna vaccine. Official figures show that more than 30,000 people have received the first shot of the Pfizer or Moderna vaccines in Norway since the end of December.

According to the Norwegian Medicines Agency, an average of 400 people die each week in nursing homes and long-term care facilities nationwide.

The Scandinavian country of 5.3 million people has confirmed more than 58,000 cases of COVID-19 since the start of the pandemic, including at least 521 deaths, according to the latest data from the Norwegian Institute of Public Health.