In the United States, there are six stages to vaccine development, which is why a single vaccine can take years or even decades to produce. In contrast, the first COVID-19 vaccine was produced and approved by the U.S. Food and Drug Administration, just 11 months after a complete genetic blueprint of a case of the virus was released in January 2020, making the COVID-19 vaccines the fastest ever created.

Since the first approvals, vaccines have seen a whirlwind of a year. Here’s a recap of both the milestones and setbacks we’ve seen in 2021.

Stage 1: Exploratory stage

The exploratory stage is composed of basic laboratory research and often lasts two to four years. Understanding viruses by sequencing their genome is becoming an important step in creating more effective vaccines. The genomic sequence of the COVID-19 virus was published online within days of it being first identified in China. Early public access to the data was a significant step as it immediately set pharmaceutical companies around the world into motion with formulating treatments, as well as preventative measures such as vaccines.

Stage 2: Preclinical stage

The preclinical stage involves ensuring that a compound is a good vaccine candidate through tests conducted on human cell culture and animals. One of the innovations that came into use for the COVID-19 pandemic was the mRNA vaccine. The idea of using mRNA in vaccines resulted from an experiment first conducted in 1987. But, for a long time, mRNA was seen as too unstable and expensive to use for drugs or vaccines. The first mRNA vaccine fully approved by the FDA was the COVID-19 vaccine made by Pfizer/BioNTech.

Stage 3: Clinical development

Clinical development is a three-stage process that takes an average of six to seven years to complete. Early- and mid-stage testing focus on gaining preliminary evidence for vaccine safety. The late-stage testing is the large trial equivalent of the early- and mid-stage testing with hundreds or thousands of participants and involves more focus on efficacy. Pfizer’s late-stage testing, for example, enrolled more than 43,000 participants. The last stage prior to FDA approval confirms the dosage level, identifies side effects and contradictions (situations where the vaccine should not be given to people with certain conditions) and compares results against other medications and vaccines.

Apart from the vaccines awaiting approval or already in use, there are 110 COVID-19 vaccines still in various stages of clinical trials. Those still in development will be the second generation of vaccines that are meant to be cheaper, easier to distribute and more effective against COVID-19 variants.

Stage 4: Regulatory review and approval

While Moderna and Johnson & Johnson are still on emergency use authorization, or EUA, Pfizer received full FDA approval Aug. 23, 2021, for those 16 years and older; it is the only COVID-19 vaccine so far that has received full FDA approval in the United States. EUA remains for kids ages 12 through 15 years old, and in October 2021, EUA was expanded for children ages 5 through 11.

Pfizer’s full FDA approval took a little more than three months from the time of its application, a process that usually takes nearly a year. The agency had to review hundreds of thousands of pages of data from Pfizer, evaluate manufacturing and facility information to ensure that vaccine production is consistently in line with the quality seen in clinical trials and analyze the benefits and risks for the vaccine’s intended population. Full FDA approval is seen as an important step after EUA because it provides support for comprehensive vaccine mandates and helps abate vaccine hesitancy.

Stage 5: Manufacturing

Since the approval and rollout of vaccines, the world has seen vaccine hoarding by first world countries and a lack of vaccine supply in developing nations. Notably, in the spring when the delta variant was first spreading and India’s COVID-19 cases skyrocketed, the Biden administration refused to lift the embargo on exporting vaccine supplies.

In total, more than 4.33 billion people have received at least one dose of a COVID-19 vaccine, but the proportion of people vaccinated is much higher in wealthier regions. For example, only 7.3% of Africans are fully vaccinated against COVID-19 in contrast to 58% in the United States and Europe.

Low- and middle-income countries have also borne an outsized  burden of disease. In May 2021, nearly 31% of recorded deaths from COVID-19 occurred in poor and lower-middle income countries, up from 9.3% the month before. This number is likely higher now and should be expected to increase due to higher vaccinations in wealthy countries. Meanwhile, only 14% of the 1.8 billion doses promised by many wealthy countries promised to low- and middle-income countries through the vaccine-sharing arrangement Covax had been delivered as of early November 2021.

The spring and summer also saw the campaign to temporarily waive intellectual property protection on coronavirus vaccines in order to boost vaccine supply for low income nations. The campaign was initiated by India and South Africa and was backed by more than 100 other countries, along with the World Health Organization and the United Nations AIDS charity, but has thus far been unsuccessful.

Stage 6: Quality control

The U.S. vaccine safety system and the FDA continually monitor the safety, effectiveness and availability of vaccines in the United States. In April 2021, as the country was approaching 150 million vaccinations, the FDA recommended a pause in the use of the Johnson & Johnson vaccine after a rare blood-clotting disorder occurred in six women who got the vaccine, one of whom died. The brief suspension coincided with the number of shots administered in the U.S. falling by 11% in the same week. Although the pause was lifted 11 days later, it may have hurt the vaccination process by increasing vaccine hesitance.

Vaccination rates have slowly plateaued and decreased in the United States, catalyzed partly by concerns over side effects. Other documented reasons for anti-vaccination sentiment include a history of medical mistreatment among Black Americans, distrust in the unusually fast production and approval of COVID-19 vaccines and distrust in the government. Building public trust in COVID-19 vaccines and the vaccination process is one of the many goals governments around the world have been focusing on in 2021, and this will likely continue into 2022.

Meanwhile, the recently identified omicron variant is potentially even more infectious than the delta variant. Already, governments around the world are instituting new measures, with the potential for a return to lockdowns in the future.

From emergency approval to manufacturing and distribution, the COVID-19 vaccines have come a long way this year in protecting those we love and fighting back against the pandemic. But with the advent of yet another winter season, a new variant and the continuation of anti-vaccination sentiment, there will still be a lot of work needed in 2022 to fully bring the world back to normal.