All three COVID-19 vaccines from Pfizer, Moderna, and Johnson & Johnson are highly effective at preventing serious illness and death from COVID-19. To get as many people vaccinated as quickly as possible, public health officials recommend getting whichever vaccine is available to you soonest. 

Different COVID-19 Vaccines (CDC)
As COVID-19 vaccines are authorized and then recommended for use in the United States, it will be important to understand what is known about each vaccine. CDC will provide information on who is and is not recommended to receive each vaccine and what to expect after vaccination, as well as ingredients, safety, and effectiveness.

Comparing the COVID-19 Vaccines: How Are They Different? (Yale Medicine)
Although each COVID-19 vaccine is unique, all of them may help with herd immunity.

Understanding How COVID-19 Vaccines Work (CDC)
COVID-19 vaccines help our bodies develop immunity to the virus that causes COVID-19 without us having to get the illness. Different types of vaccines work in different ways to offer protection. But with all types of vaccines, the body is left with a supply of “memory” T-lymphocytes as well as B-lymphocytes that will remember how to fight that virus in the future.

Why Do You Need Two Doses for Some COVID-19 Vaccines? (Healthline)
Researchers found that the Pfizer-BioNTech and Moderna vaccines provoke a relatively weak immune response when given as just one dose. However, there was a stronger immune response when a second dose was added. Basically, the first dose of the vaccine starts the process of building up protection. The second dose works to greatly reinforce this protection. Here’s an analogy to help explain this: You and a friend are trying to move a heavy table across a room. Between the two of you, you’re able to get it partway there. Then, another couple of friends jump in to help, and you’re all able to move it the rest of the way.

The Pfizer and Moderna vaccines are messenger RNA (mRNA) vaccines that trigger the body’s immune system to produce antibodies that protect people from getting infected when exposed to COVID-19. mRNA vaccines do not alter or affect your DNA. The Pfizer and Moderna vaccines both require two doses administered three to four weeks apart to be fully effective, and need to be stored at very cold temperatures. 

Understanding mRNA COVID-19 Vaccines (CDC)
mRNA vaccines are a new type of vaccine to protect against infectious diseases. To trigger an immune response, many vaccines put a weakened or inactivated germ into our bodies. Not mRNA vaccines. Instead, they teach our cells how to make a protein—or even just a piece of a protein—that triggers an immune response inside our bodies. That immune response, which produces antibodies, is what protects us from getting infected if the real virus enters our bodies.

Understanding COVID-19: How mRNA Vaccines Work (Yale Medicine)
What is RNA? How do mRNA vaccines work, and how effective are they? What does this mean for the COVID-19 pandemic? In a short video, Yale experts Saad Omer, Onyema Ogbuagu, and Akiko Iwasaki explain the science.

How Moderna’s Vaccine Works (NY Times)
Moderna, a Massachusetts-based vaccine developer, partnered with the National Institutes of Health to develop and test a coronavirus vaccine known as mRNA-1273. A clinical trial demonstrated that the vaccine has an efficacy rate of 94.1 percent in preventing Covid-19.

How the Pfizer-BioNTech Vaccine Works (NY Times)
The German company BioNTech partnered with Pfizer to develop and test a coronavirus vaccine known as BNT162b2, the generic name tozinameran or the brand name Comirnaty. A clinical trial demonstrated that the vaccine has an efficacy rate of 95 percent in preventing Covid-19.

The Johnson & Johnson vaccine is a viral vector vaccine, which uses a harmless version of the virus to train the immune system to fight COVID-19. Viral vector vaccines are commonly used for many infectious diseases. The Johnson & Johnson vaccine requires only one dose, and can be stored at normal refrigerated temperatures. 

Understanding Viral Vector COVID-19 Vaccines (CDC)
Viral vector vaccines use a modified version of a different virus (the vector) to deliver important instructions to our cells. For COVID-19 viral vector vaccines, the vector (not the virus that causes COVID-19, but a different, harmless virus) will enter a cell in our body and then use the cell’s machinery to produce a harmless piece of the virus that causes COVID-19. This piece is known as a spike protein and it is only found on the surface of the virus that causes COVID-19.

How the Johnson & Johnson Vaccine Works (NY Times)
Johnson & Johnson is testing a coronavirus vaccine known as JNJ-78436735 or Ad26.COV2.S. Clinical trials showed that a single dose of the vaccine had an efficacy rate of 72 percent in the United States, and a lower efficacy in countries where more contagious variants are widespread.