Pfizer vs. Moderna: Which is the better vaccine?

A breakdown of the two leading COVID vaccine candidates

The past two weeks has brought promising news regarding COVID vaccine candidates. You may have heard about both the Pfizer/BioNTech and Moderna vaccines and their positive results thus far in trials. This post breaks down what we know so far about the vaccines, the differences between the two, and what we have yet to find out.

What is an mRNA vaccine?

Both the Pfizer/BioNTech and Moderna vaccine candidates are what are known as mRNA vaccines. This is a novel type of vaccine; there are no vaccines on the market currently that utilize this technology. This is a result of mRNA being relatively unstable and difficult to transport to cells. Pfizer/BioNTech and Moderna have developed ways to deliver the mRNA to cells without the molecule getting degraded along the way.

Typically, vaccines use live/attenuated forms of the virus (the influenza nasal spray, MMR, smallpox) or inactive forms of the virus (the influenza shot, polio, rabies). Other vaccines utilize subunits of the virus, such as proteins or sugars that are distinct to the virus. Our bodies then recognize these subunits and mount an immune response to them (HPV, Hepatitis B). But, how exactly do mRNA vaccines work?

Recall the central dogma:

mRNA vaccines deliver mRNA, the second step of this process, to the cells to be translated into proteins. In the case of COVID, the vaccines are delivering the mRNA sequence associated with the spike protein, which our cells read and translate into the spike protein itself. Our immune systems then recognize this spike protein as foreign and mount an immune response to it.

Part of this immune response involves creating antibodies. When the body recognizes the spike protein as foreign, it creates antibodies to fight off COVID infections. These antibodies are specific to the particular pathogen (disease causing agent) it is being created for. So, when someone comes into contact with COVID who has received the vaccine, they already possess antibodies to fight off the infection. Similarly, someone can develop antibodies if they have already been infected with the virus through the same mechanism.

So, what’s the deal with Pfizer and Moderna?

Both Pfizer and Moderna have separately and successfully developed these mRNA vaccines for COVID. The table below breaks down some key differences between the two vaccines.

What do we still need to know?

Moderna has revealed some data about the diversity of their testing population both in age and ethnicity. It is unclear so far whether Pfizer has tested the same diversity of patients, but this would be necessary in evaluating the efficacy of the vaccine across populations. Also interesting to note is how this vaccine works for those who have already contracted the virus: will it solely boost immunity, or will the response lead to an overactive immune response that causes serious side effects?

Both companies are hoping to get more safety data in the coming weeks to determine whether there are any serious adverse effects of the vaccines. This will be important to know, as if there are any significant reactions to either vaccines, it could put a stopper on FDA approval.

It will also be interesting to note how the efficacy data changes in the coming weeks, and if the percentages stay at their current levels. Currently, Moderna is at a 94.5% efficacy because of 95 patients who tested positive for COVID-19 in the 30,000 person trial, 5 were in the vaccine group compared to the 90 in the placebo group. Pfizer did not reveal its exact numbers. As more data is acquired, we will know more about how accurate these efficacy statistics are.

Lastly, given the speed of these trials, we have yet to know how long immunity from these vaccines last. Will we need yearly vaccines as we do for the flu? Only time and longitudinal data will tell for these vaccines.

**If you want to learn more about antibodies, here is a fun short video on how they work: