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.

California's top epidemiologist has recommended pausing the administration of COVID-19 vaccines from Moderna's lot 041L20A due to "possible allergic reactions" that are under investigation.

"A higher-than-usual number of possible allergic reactions were reported with a specific lot of Moderna vaccine administered at one community vaccination clinic," Dr. Erica Pan, state epidemiologist for the California Department of Public Health, said in a statement late Sunday night. "Fewer than 10 individuals required medical attention over the span of 24 hours. Out of an extreme abundance of caution and also recognizing the extremely limited supply of vaccine, we are recommending that providers use other available vaccine inventory and pause the administration of vaccines from Moderna Lot 041L20A until the investigation by the CDC, FDA, Moderna and the state is complete."

The California Department of Health said all of the affected individuals appeared to be experiencing "a possible severe allergic reaction during the standard observation period," which the U.S. Centers for Disease Control and Prevention says some people have experienced when receiving a COVID-19 vaccine.

More than 330,000 doses from that specific lot have been distributed to 287 providers across the state, with the shipments arriving between Jan. 5 and 12, according to the California Department of Public Health, which said it has not been notified of any other cluster or individual events related to this lot.

When asked for comment, a spokesperson for Moderna told ABC News: "Moderna acknowledges receiving a report from the California Department of Public Health (CDPH) that a number of individuals at one vaccination center in San Diego were treated for possible allergic reactions after vaccination from one lot of Moderna’s COVID-19 Vaccine. The company is fully cooperating with CDPH in investigating these reported adverse events. Consistent with the statement from CDPH, at this point Moderna is unaware of comparable adverse events from other vaccination centers which may have administered vaccines from the same lot."

ABC News' Sony Salzman contributed to this report.