In November 2021, Public Health England reported a possible but extremely small reduction in effectiveness against symptomatic disease from the Delta sublineage AY.4.2 at longer intervals after the second dose.

[119] The UK's Medicines and Healthcare products Regulatory Agency (MHRA) advised on 9 December 2020 that people who have a history of "significant" allergic reaction should not receive the Pfizer–BioNTech COVID‑19 vaccine.

[141] At this stage, it takes only four days to go from mRNA and lipids to finished vials, but each lot must then spend several weeks in deep-freeze storage while undergoing verification against 40 quality-control measures.

[138] Therefore, all doses administered in the Americas outside of the United States before that point in time required at least two transatlantic flights (one to take DNA to Europe and one to bring back finished vaccine vials).

The head of Indonesia's Bio Farma Honesti Basyir said purchasing the vaccine is out of the question for the world's fourth-most populous country, given that it did not have the necessary cold chain capability.

[171]: 2 He then came across a submission from Hong Kong-based researchers on the website of the medical journal The Lancet in which they discussed a cluster of pneumonia associated with coronavirus and an indication of person-to-person transmission that had affected a family that had recently returned from Wuhan.

[174] The authors of the submission were of the opinion that they were observing the early stages of an epidemic,[171]: 5–7 While no infectious disease expert Şahin did some quick calculations based on Wuhan's population and transport links and came to the conclusion that if this virus was possible of person-to-person transmission then it could cause a morality rate somewhere between 0.3 and 10 out of every 100 inflected people to give a best case scenario of two million deaths worldwide.

[171]: 29  He was assisted in his work by the SARS-CoV-2 genetic sequences having been previously published on 11 January 2020[171]: 120  by Edward C. Holmes in association with Zhang Yongzhen, a professor at the Chinese Center for Disease Control and Prevention on open-source website Virological.org.

[171]: 34–37 At the board meeting the next day Şahin received permission to spend over the next weeks a limited amount of money that the company and its 1,300 personnel investigating the development of a vaccine, after which they would reevaluate whether to continue.

[171]: 41, 165  The board then considered whether to build up their capability to fully manufacture, document, sell and distribute any potential vaccine they decided that this would take too long and it would be better to partner with a pharma giant.

[171]: 49–50  Crucial to the delivery of an mRNA vaccine to its cellular destination via an injection into a human muscle was the availability of a suitable wrapper made of lipid nano particles to protect it from the body's enzymes.

[171]: 56–60 For the vaccine to work it needed to deliver a stable accurate replica of the virus's spike protein so that the body's immune system could recognize and react to COVID‑19 if they became infected.

[171]: 72–75  In developing a stable replica, the team was assisted by advice from Barney S. Graham who had been studying the MERS virus, which was approximately 54% identical to the uploaded COVID-19 genetic code.

This was then poured into a 50 ml bag, frozen to minus 70 degrees Celsius and dispatched by a waiting car to Polymun to be combined with the lipids, a process that was to followed by the rest of the 20 candidates.

[171]: 122 Once the first vials containing the lipid wrapped mRNA candidates were revied back in Mainz on 9 March[171]: 129  a team led by Annette Vogel began testing them to determine which using at various dosage amounts induced the best immune responses, first in glass dishes and then at a separate location, in mice.

[171]: 129  The blood was analyzed by a team led by Lena Kranz and Mathias Vormehr to check to see if the mice's T-cells reacted and carried out the required immune response.

[179] The release of news of the partnership bought BioNTech publicity that resulted the company receiving letters and telephone calls containing racists views and often death threats.

[188] Pfizer CEO Albert Bourla said he decided against taking funding from the US government's Operation Warp Speed for the development of the vaccine "because I wanted to liberate our scientists [from] any bureaucracy that comes with having to give reports and agree how we are going to spend the money in parallel or together, etc."

[49] The study expanded during mid-2020 to assess efficacy and safety of BNT162b2 in greater numbers of participants, reaching tens of thousands of people receiving test vaccinations in multiple countries in collaboration with Pfizer and Fosun.

[203] A research letter published in JAMA reported that the vaccines appeared to be safe for immunosuppressed organ transplant recipients, but that the resulting antibody response was considerably poorer than in the non-immunocompromised population after only one dose.

The US Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices (ACIP) approved recommendations for vaccination of those aged sixteen years or older.

[248][249] Following the EUA issuance, BioNTech and Pfizer continued the Phase III clinical trial to finalize safety and efficacy data, leading to application for licensure (approval) of the vaccine in the United States.

[260] In June 2021, the UK Medicines and Healthcare products Regulatory Agency (MHRA) came to a similar decision and approved the use of the vaccine for people twelve years of age and older.

[275][276] In July 2021, the U.S. Food and Drug Administration (FDA) granted priority review designation for the biologics license application (BLA) for the Pfizer–BioNTech COVID-19 vaccine with a goal date for the decision in January 2022.

On 29 August 2021, Israel's coronavirus czar announced that Israelis who had not received a booster shot within six months of their second dose would lose access to the country's green pass vaccine passport.

[284] The US Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) authorized the use of an additional mRNA vaccine dose for immunocompromised individuals at that time.

[82][83] Scientists and the WHO noted in August 2021, the lack of evidence on the need for a booster dose for healthy people and that the vaccine remains effective against severe disease months after administration.

[70] In October 2021, the US Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) authorized the use of either homologous or heterologous vaccine booster doses.

[338] Accounts of how Pfizer's got its way into a large deal to provide 1.8 billion doses of its vaccine to the European Union were described by The New York Times as "a striking alignment of political survival and corporate hustle".

In early 2023, Belgian prosecutors began investigating European Commission President Ursula von der Leyen and Pfizer CEO Albert Bourla.