Here you have links to medical and scientific journals. Most of these links take you directly to their COVID-19 section or search page. Although practically all of these journals normally charge to access articles, most of them (at least during the first year of the pandemic) had no paywall (they allowed free access) to articles on COVID-19 or coronaviruses. Most however required you to register. Many pages and articles on COVID-19 are still (as of February 7, 2025) available without a subscription.
Other publications or gateways to journals
CDC: Novel Coronavirus Reports
(the website is being modified as of February 7, 2025)