Part 1: Introduction to mRNA Vaccines

1.3. Getting to Humans

As one could imagine, high safety and efficacy standards need to be met for a vaccine to be used on humans nowadays. The validation process has been refined for more than a century, and usually takes 10-15 years and more than a billion dollars for a single vaccine.

First, the target needs to be known. This not only covers the disease/condition to be treated, but also the underlying mechanisms - obtaining that knowledge is usually not considered as part of the pipeline (and thus not in the 10-15 years estimate mentioned above). The real journey starts at finding and choosing the very specific step of the mechanism a vaccine should target - the antigen that your body should recognize and eliminate after immunization. Finding and producing such an antigen usually takes 2-5 years and costs $10-20 million, but bioinformatics and the advantages of mRNA vaccines can bring these estimates significantly down, as we’ll see in Part 2.

Then, vaccines go through a series of pre-clinical and clinical trials to ensure safety and efficacy. Testing starts in animals and proceeds to healthy volunteers and then patients. The three phases of human trials are the most costly - they usually take 4-8 years and have a price tag of 500 million to a billion dollars.

Before even thinking about humans, promising results have to be shown on animals. While mice are the first animal model that comes to mind (and one of the most standardized), the animal species should be carefully chosen in each case. For example, ferrets are sometimes used to model influenza because their infection pattern matches that of humans, and no modifications need to be made to the virus to infect them (which is not the case for mice). The animal testing phase, also referred to as pre-clinical, can take 1-2 years (heavily depends on the time it takes for the disease to cause symptoms, immunization, and other factors) and around $50-100 million.

After that, vaccines seeking FDA approval enter the three official phases of human testing. Compared to other types of medicine, vaccine trials have about an order of magnitude more total subjects in the later phases (15.000 - 100.000 compared to around 5.000), and accept a much lower incidence of adverse effects, as vaccines are mostly given as preventative measures to healthy individuals.

Phase I mainly studies the safety of a proposed vaccine, as well as the maximum tolerable dose. Healthy volunteers are given different doses to establish the healthy range, as well as possible side effects. Some volunteers will receive placebos to establish a baseline, and neither the people administering the doses nor the volunteers know which group they belong to until the study is finished. Phase I trials usually enroll 20-80 participants and are exploratory in nature. Note there’s usually no efficacy testing during this phase - the main goal is to use a small number of volunteers to get early confirmation that the vaccine is safe. During this phase, more attention is usually given to live vaccines, which tend to have a higher risk - although they don’t possess the ability to cause the target condition (disease), mRNA vaccines are still very closely monitored given their novelty.

During Phase II trials, the focus started shifting towards ensuring the vaccines work - while safety is carefully monitored and trials end if any major danger is detected, effectiveness takes the spotlight. Phase II demonstrates the vaccine generates the intended immune system response while allowing researchers to define its optimal dose, initial schedule, and safety profile. Usually, groups receiving different doses with different schedules (and a placebo group) will be statistically compared, with one of the most important criteria being the level of immune response, usually measured by the level of antibodies or even direct immune system response.

With promising results from the first two phases, Phase III may begin. This phase is similar to Phase II in design, but applied at a much bigger scale (15.000 - 100.000 participants). Phase III looks to convincingly establish effectiveness, safety, and side effects, including rare ones - safety data for vaccines primarily comes from Phase III trials.

The trials are held to the highest rigors and usually follow the most ‘ideal’ setups one could ask for from a trial - from advanced control groups and double-blinding (neither the participants nor the researchers are aware whether the participant received the real vaccine or a placebo during the trial) to rigorous statistical validation of results. Once Phase III trials are done, licensing the vaccine may take an additional 1-2 years.

Multiple agencies are involved in the licensing process, with the United States Food and Drug Administration (FDA) and the World Health Organization (WHO) being the most notable for US-based processes. Each organization has specific requirements needed for a review - in communication with the FDA, vaccine manufacturers apply for a Biologics License Application. The FDA’s review team employs a multidisciplinary approach when reviewing the efficiency and efficacy of vaccines, ultimately performing a risk assessment. Since phase III only has tens to hundreds of thousands of participants, the FDA and drug companies still monitor vaccines once they’re approved and commercially produced, which is sometimes described as ‘Phase IV. The surveillance phase is also essential, as it can reveal very rare side effects. For example, the H1N1 influenza vaccine Pandemrix was mostly restricted in children and adolescents after Phase IV data showed it raises the risk of developing narcolepsy. Phase IV takes at least (but usually over) 2-5 years and 20 million dollars. In total, the typical vaccine testing and approval process takes 8 to 17 years, with total costs around 560-1,200 million dollars. Even with those costs, only around 6% of vaccines used in preclinical studies make it to the market.

The first clinical trial of mRNA vaccines took place in 2013, when CureVac conducted a Phase I trial on an mRNA vaccine aiming to treat a rabies virus glycoprotein. More notably, in 2020, two mRNA vaccines - mRNA-1273 by Moderna and BNT162b2 by Pfizer/BioNTech - were approved by the FDA and used during the COVID-19 outbreak. The successful application of the two vaccines marked an important milestone in using vaccines as a rapid/emergency response against infectious disease outbreaks. It has also caused an explosion in the development of mRNA vaccines for a variety of targets, from viruses to bacteria and even parasites.

Most safety and efficacy data for mRNA vaccines used on humans are, unsurprisingly, on COVID-19 vaccines. In a meta-analysis of data from 25 trials, it was found that mRNA vaccines did not have a higher incidence of serious adverse effects compared to placebos. Moreover, while efficacy in preventing symptomatic SARS-CoV-2 infections was statistically the same, a general trend of mRNA vaccines being more effective than protein subunit vaccines, viral vector vaccines, inactivated vaccines, and DNA vaccines was noted. However, mRNA vaccines showed more severe effects compared to inactivated virus vaccines, DNA vaccines, viral vector vaccines, and protein subunit vaccines. Notably, the majority of severe adverse reactions of COVID-19 vaccinations are associated with cardiac complications and affect primarily male adolescents. Moreover, mild anaphylactic reactions to the vaccines have been reported, with rates ranging from 2.5 per million for the Moderna vaccine and 2.2 per million for the Pfizer-BioNTech vaccine. While the rates are significantly higher than those from traditional vaccines, researchers proposed that the rates are related to the presence of PEGylated lipids used in LNPs: around 40% of the population is reported to have anti-PEG antibodies. It’s important to remain cautious about LNPs and mRNA delivery vehicles in general, as they could cause potential hazards such as inflammation and cytotoxicity.

Side effects sound scary, but it’s important to note that we’re aware of them because extreme measures are taken to measure and assure vaccine safety. While caution is warranted, COVID-19 mRNA vaccines are an astounding success. Recent estimates show that the Pfizer-BioNTech and Moderna COVID-19 mRNA vaccines saved 7.4-23.6 million life-years.