This highly informative review summarizes studies addressing the immune response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), including available vaccines and their trials. The following are 10 key points summarizing its findings:

1. Seasonal human coronaviruses (alpha and beta) are estimated to cause up to one third of community-acquired upper respiratory tract infection. Immunity to seasonal human coronaviruses tends to be short in duration, lasting from 80 days to a few years, with re-infections documented. Whether the immune response to SARS-CoV-2 follows a similar pattern is unclear.
2. There is no pre-existing immunity to SARS-CoV-2 in the population, except through cross-reactivity (shared viral antigens or epitopes) with other coronaviruses. Whether pre-existing immunity to common human seasonal coronaviruses might offer some degree of cross-protection is unknown.
3. The humoral immune response to SARS-CoV-2 is mediated by antibodies directed to the viral surface spike glycoprotein, for which its subunit has a receptor-binding domain that allows viral entry into cells through binding of angiotensin-converting enzyme 2 (ACE2) receptors.
4. IgG titers increase during the first 3 weeks and begin to decrease 8 weeks after infection. Antibody titers also correlate with severity of infection, suggesting that SARS-CoV-2 humoral immunity might not be long-lasting in individuals with mild disease.
5. Differences in antibody titer generation related to age and sex have been reported, with older patients and men having significantly higher IgG titers than younger patients and women, respectively.
6. Inborn genetic mutations that disrupt type I interferon responses have been identified as risk factors for severe coronavirus 2019 (COVID-19), suggesting that innate immune response plays an important role in protection against SARS-CoV-2.
7. Regarding cellular immunity, the CD4+ T-cell response predominantly consisted of T-helper-1 (Th1) cells, characterized by high concentrations of IFNγ secretion. CD8+ T-cell responses specific to SARS-CoV-2 produced IFNγ and tumor necrosis factor α, with a pattern towards structural proteins and skewed towards a Th1 response.
8. Recognition of SARS-CoV-2 antigens by pre-existing and cross-reactive T cells created during previous infection with human coronaviruses might also contribute to the frequent presence of T cells reactive to SARS-CoV-2 in patients with COVID-19.
9. The Food and Drug Administration has issued guidance stating that a COVID-19 vaccine would have to protect ≥50% of vaccinated people to be considered efficacious.
10. There are nine vaccines for SARS-CoV-2 undergoing phase 3 trials across the world, and consist of either an adenoviral vector, mRNA based, or inactivated virus.

Clinical Topics: COVID-19 Hub, Prevention

Keywords: Angiotensin-Converting Enzyme Inhibitors, Antigens, Viral, Coronavirus, COVID-19, Cross Protection, Immunity, Humoral, Interferon Type I, Membrane Glycoproteins, Peptidyl-Dipeptidase A, Primary Prevention, Receptors, IgG, RNA, Messenger, severe acute respiratory syndrome coronavirus 2, Th1 Cells, Vaccine Potency

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