A COVID‑19 vaccine is designed to induce immunity against SARS-CoV-2, the virus responsible for coronavirus disease 2019 (COVID-19). COVID-19 vaccines help reduce the risk of severe illness, hospitalization and death from the virus.
COVID‑19 vaccines were developed at an unprecedented pace to tackle the COVID-19 pandemic, with the first clinical trials beginning in March 2020. Before approval, vaccines underwent the standard three phases of clinical trials, although phases were conducted in parallel to accelerate development. Vaccines have been developed based on both classical technologies (using inactivated virus or protein subunit) and novel platforms (mRNA or viral vector-based).
Prior research on coronaviruses causing severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS) accelerated the development of various vaccine platforms in early 2020. The 2023 Nobel Prize in Physiology or Medicine was awarded to Katalin Karikó and Drew Weissman for the development of effective mRNA vaccines against COVID‑19.
Major vaccines include the Pfizer–BioNTech mRNA vaccine, Moderna mRNA vaccine, and the Novavax protein subunit vaccine. With the emergence of new SARS-CoV-2 variants, the original vaccines—particularly Pfizer–BioNTech and Moderna vaccines—have been updated. These "variant-adapted" vaccines are offered as booster doses. The immunity from the vaccines also wanes over time, requiring people to get boosters to maintain protection.
Common side effects of COVID‑19 vaccines include soreness, fatigue, headache, myalgia (muscle pain), and arthralgia (joint pain), which resolve without medical treatment within a few days. COVID‑19 vaccination is safe for people who are pregnant or are breastfeeding.
The COVID‑19 vaccines are widely credited for their role in reducing the spread of COVID‑19 and reducing the severity and death caused by COVID‑19. The Australian-based medical journal Journal of Paediatrics and Child Health estimated that between 14.4 and 19.8 million deaths were prevented by the vaccine. Many countries implemented phased distribution plans that prioritized those at highest risk of complications, such as the elderly, and those at high risk of exposure, such as healthcare workers. By December 2020, more than 10 billion vaccine doses had been preordered, with about half of the doses purchased by high-income countries comprising 14% of the world's population. As of August 2024, over 13 billion doses of COVID‑19 vaccines have been administered worldwide.
Before COVID‑19, a vaccine for an infectious disease had never been produced in less than several years – and no vaccine existed for preventing a coronavirus infection in humans. However, vaccines have been produced against several animal diseases caused by coronaviruses, including infectious bronchitis virus in birds, canine coronavirus, and feline coronavirus.
Previous projects to develop vaccines for viruses in the family Coronaviridae that affect humans have been aimed at severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS). Vaccines against SARS and MERS have been tested in non-human animals.
According to studies published in 2005 and 2006, the identification and development of novel vaccines and medicines to treat SARS were priorities for governments and public health agencies worldwide at the time. There is no cure or vaccine proven to be safe and effective against SARS in humans. There is also no proven vaccine against MERS. When MERS became prevalent, it was believed that previous SARS research might provide a useful template for developing MERS vaccines. As of March 2020, there was one (DNA-based) MERS vaccine that completed Phase I clinical trials in humans, and three others in progress, all being viral-vectored vaccines: two based on adenoviruses and one on MVA.
Vaccines that use an inactive or weakened virus that has been grown in eggs typically take more than a decade to develop. In contrast, mRNA is a molecule that can be made quickly, and research on mRNA to fight diseases was begun decades before the COVID‑19 pandemic by scientists such as Drew Weissman and Katalin Karikó, who tested on mice. Moderna began human testing of an mRNA vaccine in 2015. Viral vector vaccines were also developed for the COVID‑19 pandemic after the technology was previously cleared for Ebola.
The initial focus of SARS-CoV-2 vaccines was on preventing symptomatic, often severe, illness. Most of the first COVID‑19 vaccines were two-dose vaccines. Exceptions were the single-dose vaccines Convidecia and the Janssen COVID‑19 vaccine, and vaccines with three-dose schedules, Razi Cov Pars and Soberana.
In July 2021, at least nine different technology platforms were under research and development to create an effective vaccine against COVID‑19. The coronavirus spike protein (S protein), which the virus uses to enter the cell, induces a strong immune response and is therefore the primary target of vaccines.
However, other coronavirus proteins were also being investigated for vaccine development, for example, the nucleocapsid proteins, because they also induce a robust T-cell response, while their genes are more conserved and recombine less frequently. Future generations of COVID‑19 vaccines targeting more conserved genomic regions could be used to treat future variations of SARS-CoV-2, or any similar coronavirus epidemic/pandemic.
Platforms developed in 2020 involved nucleic acid technologies (mRNA and DNA), non-replicating viral vectors, peptides, recombinant proteins, live attenuated viruses, and inactivated viruses.
Many vaccine technologies developed for COVID‑19 use "next-generation" strategies for precise targeting of COVID‑19 infection mechanisms. Several of the synthetic vaccines use a 2P mutation to lock the spike protein into its prefusion configuration, stimulating an adaptive immune response to the virus before it attaches to a human cell.
Several COVID‑19 vaccines, such as the Pfizer–BioNTech and Moderna vaccines, use RNA to stimulate an immune response. When introduced into human tissue, the vaccine contains messenger RNA (mRNA), which causes cells to express the SARS-CoV-2 spike protein. This teaches the immune system to identify and destroy the corresponding pathogen. RNA vaccines often use nucleoside-modified messenger RNA. The delivery of mRNA is achieved by encapsulating the molecule in lipid nanoparticles, which protect the RNA strands and help their uptake into the cells.
mRNA vaccines were the first COVID‑19 vaccines authorised in the United Kingdom, the United States, and the European Union. Authorized vaccines of this type include the Pfizer–BioNTech and Moderna vaccines. The CVnCoV mRNA vaccine from CureVac failed in clinical trials.
Severe allergic reactions to the mRNA vaccines are rare. In December 2020, 1,893,360 first doses of Pfizer–BioNTech COVID‑19 vaccine administration resulted in 175 cases of severe allergic reactions, of which 21 were anaphylaxis. For 4,041,396 Moderna COVID‑19 vaccine dose administrations in December 2020 and January 2021, only ten cases of anaphylaxis were reported. Lipid nanoparticles (LNPs) were most likely responsible for the allergic reactions.