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SARS-CoV-2, the virus behind COVID-19, is an enveloped coronavirus with spike proteins measuring about 100 nanometers, responsible for the global pandemic that began in 2019.

About this subject

SARS-CoV-2 belongs to the Coronaviridae family, which includes other human coronaviruses such as those causing SARS and MERS. Its structure comprises a single-stranded positive-sense RNA genome enclosed in a lipid envelope derived from the host cell membrane. The spike (S) proteins project from the surface and are responsible for binding to the ACE2 receptor on human cells, primarily in the respiratory tract. The spike glycoprotein is the main target of vaccines, as neutralizing antibodies can block it.

Since its identification in Wuhan, China, in December 2019, the virus has undergone multiple mutations, generating variants such as Alpha, Delta, and Omicron, each with distinct transmissibility and immune escape characteristics. Infection can be asymptomatic or cause mild to severe symptoms, including severe acute respiratory syndrome (SARS). The pandemic resulted in over 770 million confirmed cases and 7 million deaths by 2025, according to WHO.

Viral replication occurs in the host cell cytoplasm. The viral RNA is translated into nonstructural proteins that form the replication complex. Production of new viral particles involves assembly in the endoplasmic reticulum and Golgi complex, followed by release via exocytosis. The virus can survive on surfaces for hours or days, depending on material and environmental conditions, although aerosol transmission is predominant.

mRNA vaccines (Pfizer-BioNTech, Moderna), viral vector vaccines (AstraZeneca, Janssen), and recombinant protein vaccines (Novavax) were developed in record time. All induce immune responses against the spike protein. The emergence of Omicron reduced efficacy against infection, but protection against severe disease remains high with booster doses. Drugs like Paxlovid (nirmatrelvir/ritonavir) and remdesivir are used for high-risk cases.

Frequently Asked Questions

How does SARS-CoV-2 infect human cells?

The virus binds to the ACE2 receptor on cell surfaces via the spike protein. Then its membrane fuses with the cell membrane, releasing viral RNA into the cytoplasm to start replication.

What are the main coronavirus variants and why did they emerge?

Variants such as Alpha, Delta, and Omicron emerged due to random genetic mutations. Some confer advantages like increased transmissibility or partial escape from the immune response elicited by vaccines or prior infection.

Are COVID-19 vaccines effective against new variants?

Vaccines continue to protect against severe disease, hospitalization, and death, even against variants like Omicron. However, efficacy against mild infection may decrease, so booster doses with updated vaccines targeting circulating strains are recommended.