COVID-19

=== Immunity ===
{{See also|COVID-19 vaccine}}
[[File:Diagnostics-10-00453-g001.webp|thumb|Human [[antibody response]] to SARS-CoV-2 infection]]

The [[immune response]] by humans to SARS-CoV-2 virus occurs as a combination of the [[cell-mediated immunity]] and antibody production,<ref>{{#invoke:Cite web||url=https://www.ecdc.europa.eu/en/covid-19/latest-evidence/immune-responses |title=Immune responses and correlates of protective immunity against SARS-CoV-2 |date=18 May 2021 |publisher=European Centre for Disease Prevention and Control |access-date=3 June 2021}}</ref> just as with most other infections.<ref>{{#invoke:cite journal || vauthors = Vabret N, Britton GJ, Gruber C, Hegde S, Kim J, Kuksin M, Levantovsky R, Malle L, Moreira A, Park MD, Pia L, Risson E, Saffern M, Salomé B, Esai Selvan M, Spindler MP, Tan J, van der Heide V, Gregory JK, Alexandropoulos K, Bhardwaj N, Brown BD, Greenbaum B, Gümüş ZH, Homann D, Horowitz A, Kamphorst AO, Curotto de Lafaille MA, Mehandru S, Merad M, Samstein RM  | title = Immunology of COVID-19: Current State of the Science | journal = Immunity | volume = 52 | issue = 6 | pages = 910–941 | date = June 2020 | pmid = 32505227 | pmc = 7200337 | doi = 10.1016/j.immuni.2020.05.002 | doi-access = free | title-link = doi }}</ref> B cells interact with T cells and begin dividing before selection into the plasma cell, partly on the basis of their affinity for antigen.<ref>{{#invoke:cite journal || vauthors = Wang Z, Muecksch F, Schaefer-Babajew D, Finkin S, Viant C, Gaebler C, Hoffmann HH, Barnes CO, Cipolla M, Ramos V, Oliveira TY, Cho A, Schmidt F, Da Silva J, Bednarski E, Aguado L, Yee J, Daga M, Turroja M, Millard KG, Jankovic M, Gazumyan A, Zhao Z, Rice CM, Bieniasz PD, Caskey M, Hatziioannou T, Nussenzweig MC  | title = Naturally enhanced neutralizing breadth against SARS-CoV-2 one year after infection | journal = Nature | volume = 595 | issue = 7867 | pages = 426–431 | date = July 2021 | pmid = 34126625 | pmc = 8277577 | doi = 10.1038/s41586-021-03696-9 | bibcode = 2021Natur.595..426W }}</ref> Since SARS-CoV-2 has been in the human population only since December 2019, it remains unknown if the [[Immunity (medical)|immunity]] is long-lasting in people who recover from the disease.<ref name="CohenJI2020Dec">{{#invoke:cite journal || vauthors = Cohen JI, Burbelo PD | title = Reinfection with SARS-CoV-2: Implications for Vaccines | journal = Clinical Infectious Diseases | date = December 2020 | pmid = 33338197 | pmc = 7799323 | doi = 10.1093/cid/ciaa1866 | s2cid = 229323810 | title-link = doi | volume = 73 | issue = 11 | pages = e4223–e4228 | doi-access = free }}</ref> The presence of neutralising antibodies in blood strongly correlates with protection from infection, but the level of neutralising antibody declines with time. Those with asymptomatic or mild disease had undetectable levels of neutralising antibody two months after infection. In another study, the level of neutralising antibodies fell four-fold one to four months after the onset of symptoms. However, the lack of antibodies in the blood does not mean antibodies will not be rapidly produced upon reexposure to SARS-CoV-2. Memory B cells specific for the spike and nucleocapsid proteins of SARS-CoV-2 last for at least six months after the appearance of symptoms.<ref name="CohenJI2020Dec" />

As of August 2021, reinfection with COVID‑19 was possible but uncommon. The first case of reinfection was documented in August 2020.<ref name="Wang-2021">{{#invoke:cite journal || vauthors = Wang J, Kaperak C, Sato T, Sakuraba A | title = COVID-19 reinfection: a rapid systematic review of case reports and case series | journal = Journal of Investigative Medicine | volume = 69 | issue = 6 | pages = 1253–1255 | date = August 2021 | pmid = 34006572 | doi = 10.1136/jim-2021-001853 |issn=1081-5589 | s2cid = 234773697 }}</ref> A systematic review found 17 cases of confirmed reinfection in medical literature as of May 2021.<ref name="Wang-2021" /> With the [[SARS-CoV-2 Omicron variant|Omicron variant]], as of 2022, reinfections have become common, albeit it is unclear how common.<ref name="abc-reinfections"/> [[SARS-CoV-2#Reinfection|COVID-19 reinfections]] are thought to likely be less severe than primary infections, especially if one was previously infected by the same variant.<ref name="abc-reinfections">{{#invoke:cite news ||title=How soon after catching COVID-19 can you get it again? |url=https://www.abc.net.au/news/health/2022-05-03/covid-19-reinfection-what-are-the-odds-of-catching-it-twice/101024180 |access-date=24 June 2022 |work=ABC News |date=2 May 2022 }}</ref>{{additional citation needed|date=July 2022}}