Chinese epidemiologists identified the first cluster of patients with pneumonia of an unknown cause in Wuhan, Hubei Province of China. This marked the beginning of the COVID-19 pandemic.

Chinese authorities confirmed they were treating dozens of cases of pneumonia of unknown etiology and informed the World Health Organization (WHO) about these cases. Researchers in China identified a new virus that had infected dozens of people.

Chinese authorities identified the disease as a new type of coronavirus, novel coronavirus or nCoV. This identification was a crucial step in understanding the nature of the outbreak.

Chinese authorities locked down Wuhan to halt the spread of the disease. This was one of the first major containment measures taken to combat the outbreak.

The WHO Director-General, Dr. Tedros Adhanom Ghebreyesus, declared the outbreak a Public Health Emergency of International Concern. The Trump administration also suspended entry into the United States by any foreign national who had traveled to China in the past 14 days.

The WHO declared COVID-19 a pandemic due to the alarming levels of spread and severity of the outbreak. This declaration highlighted the global nature of the crisis.

States in the U.S. began to shut down to prevent the spread of COVID-19. New York City's public school system shut down, and Ohio called for restaurants and bars to close. Social distancing measures were extended until the end of April 2020.

The CDC recommended that all people wear a mask when outside their homes to reduce the transmission of the virus. This was a significant shift in public health guidance.

President Trump launched Operation Warp Speed, an initiative aimed at producing a vaccine for the coronavirus as quickly as possible. This initiative played a crucial role in the rapid development and distribution of COVID-19 vaccines.

Pfizer and Moderna announced the start of their phase 3 clinical trials for their respective COVID-19 vaccines. These trials were pivotal in the eventual approval and distribution of the vaccines.

The CDC's Advisory Committee on Immunization Practices (ACIP) recommended that health care professionals and residents of long-term care facilities be prioritized for the COVID-19 vaccine. This marked the beginning of the vaccine rollout in the U.S..