What You Need to Know About the Top Four Vaccines

Published on: Apr 13, 2021fb-roundtwitter-roundemail-round
Sandra Lindsay, Coronavirus, COVID-19 vaccine, Pfizer, BioNTechGetty Images

COVID-19 pretty much shut down 2020. What's expected to put things back on track: vaccines. Right now, there are at least 8 COVID-19 vaccines in clinical development. With an additional 184 vaccine candidates in preclinical trials, as of April 13.

That seems like a lot

Sure is. But all of these vaccines are in different stages of development and approval. And with all the terms being thrown around, Phase 1, Phase 2, Phase 3, emergency use authorization, efficacy, it can be tough to figure out what all of this really means, and more importantly is it safe? So let's break it down. After researchers have identified a potential vaccine candidate they start the clinical testing process.

Preclinical testing...when scientists test the vaccine candidate on cells and then give it to animals to see if it creates an effective immune response.

Phase 1 (aka the safety trial)...when humans start to come in. A small number of people (20-100) are given the vaccine during this round. Scientists are testing for safety, best dosage amount, and to confirm that an immune response is being created. Approximately 70% of drugs will move onto the next phase.

Phase 2 (aka the expanded trial)...when hundreds of humans are given the vaccine candidate. And this time they are split into groups by health and demographics. Researchers want to know how the drug will affect different types of people. And scientists are keeping an eye out for how safe it is. Plus that the immune response is still going strong. Approximately 33% of candidates will move on to the next phase.

Phase 3 (aka the efficacy trial)…when scientists test the vaccine candidate on thousands of people to determine how well it works (hence efficacy). This is the phase we hear the most about because it provides the most safety data since it’s tested on so many more people. Scientists find less common, long-term, and rare side effects that may have missed in previous phases. And they are still testing for the tried and true immune response – which tells them whether the vaccine actually works. Approximately 25-30% of candidates move on from here.

Usually, this process can take years to complete, but because these are unusual times, researchers were able to combine phases to speed up testing thanks to things like new technology, years of research on a virus similar to COVID-19, and the financial incentives governments are providing to support this research. (Think: Operation Warp Speed.)

In the US, vaccine manufacturers need to get FDA approval before marketing and distribution can begin. And the steps for the approval process typically start after a successful Phase 3. A manufacturer presents their vaccine at “science court,” where a panel of independent experts act as a jury and pick apart the vaccine. If they're satisfied with the data, they endorse it and send it off to the FDA for final approval.

Which vaccines got the thumbs up so far?

In the US, three have gotten emergency use authorization – Pfizer/BioNTech, Moderna, and Johnson & Johnson. And across the world, there are more than a dozen vaccine candidates reportedly conducting Phase 3 trials. Here are the ones leading the pack:

Johnson & Johnson
  • Status: Approved for emergency use in the US – but in mid-April, the CDC and FDA recommended a pause on the vaccine. That’s because of six reported cases of a “rare and severe” type of blood clot after vaccination in the US. (For context, more than 6.8 million people have gotten the J&J shot.) The people who experienced the clots are all women ages 18 to 48, and symptoms occurred six to 13 days after they got their shot. One of the patients has died. Now, a number of states, including Connecticut, Ohio, New Jersey, and New York have temporarily stopped J&J vaccinations.

  • Efficacy rate: In US clinical trials it showed a 72% rating. But its protection rates in countries where the virus’s variants have become dominant showed a lower rating. (Think: 57% in South Africa.)

  • Dose: 1 dose

  • How does it work: J&J’s vaccine can be stored for up to three months at regular fridge temps – making it easier to store and ship. The vaccine uses a genetically-modified adenovirus (which typically causes flu and cold symptoms but it can’t in this case since it’s altered) and basically turns it into a delivery van. Inside the van, is double-stranded DNA – which has the instructions to create spike proteins (the protein found on the surface of the coronavirus).

  • Here’s how it goes down: After the vaccine is injected into the arm, the adenovirus enters a cell in the body to deliver its message (aka the DNA) to the cell’s nucleus (where the genes are stored). From there the cell reads the DNA and converts it into messenger RNA (mRNA). Now, the cell is vaccinated and can alert the rest of the immune system, replicate the spike protein, recognize the virus, and create antibodies to protect against coronavirus.

  • Side effects: In addition to the blood clot reports (see above), J&J’s vaccine appears to be less likely to trigger allergic reactions. And the main side effects appear to be pain at the injection site, fever, fatigue, and headache.

  • Status: Phase 2/3. Approved for emergency use in the US.

  • Efficacy rate: 95%

  • Dose: 2 doses, 3 weeks apart

  • How does it work?: Pfizer/BioNTech’s vaccine is injected into the muscle. And must be stored at -94 degrees Fahrenheit. Unlike J&J’s vaccine, it uses genetic material called mRNA instead of DNA to send an alert to the immune system. Basically, this messenger delivers a piece of the coronavirus’s genetic code to the body’s immune system. The body uses that information to build spike proteins and replicates them. After this process is complete, the immune system is now able to recognize the virus and attack it when it enters the body.

  • Side effects: No serious side effects. Some trial participants reported pain at the injection site and flu-like symptoms like fever, chills, and body aches. But UK health officials are saying people with a "significant history of allergic reactions" should hold off on getting this vaccine after two health care workers had adverse reactions after getting the first dose.

  • Status: Phase 3. Approved for emergency use in the US.

  • Efficacy rate: 94.5%

  • Dose: 2 doses 4 weeks apart

  • How does it work?: Moderna’s vaccine is injected into the muscle. And it can last 30 days in a fridge. Like Pfizer/BioNTech’s vaccine, it uses cutting-edge mRNA technology.

  • Side effects: So far, Moderna has noted that participants have felt pain at the injection site, muscle aches, and headaches.

  • Status: Expected to file for emergency use authorization in the US. Approved for emergency use in the UK, EU, Canada, and COVAX, but a number of countries have suspended vaccinations for certain age groups because of reports of rare blood clots after vaccinations.

  • Efficacy rate: 76%

  • Dose: 2 doses 4 weeks apart

  • How does it work?: Like J&J, the AstraZeneca-Oxford vaccine uses a harmless cold-causing virus that scientists have modified to include genetic material from the coronavirus. And similar to the Pfizer/BioNTech and Moderna vaccines, once injected it’ll stimulate an immune response so the system can learn to recognize the virus.

  • Side effects: Aside from blood clot concerns, no serious side effects have been reported.

  • Important note: This vaccine is the cheapest of the four. It can be stored at normal fridge temperatures. And it's easy to make. But a series of data blunders have left some doubting its ability to stand up to public and scientific scrutiny.

So what can Americans expect?

A lot. On average, the US is vaccinating 3 million Americans a day. And President Joe Biden pledged to get 200 million vaccine shots in arms during his first 100 days in office – a feat that he has until the end of day on April 29 to complete, and an upgrade from his initial promise of 100 million shots in 100 days.


While we’re seeing giant leaps and breakthroughs happen at a rapid pace in the COVID-19 vaccine race, it’s important to remember that it may take a while for the US to reach herd immunity. But hope is contagious and these vaccine developments are signaling a possible return to normal. In the meantime, it’s important to continue using safety precautions to stop the spread until more doses of the vaccine become available.

Last updated on April 13 – Updated to reflect recent changes with clinical trials, the Johnson & Johnson and AstraZeneca vaccines, and vaccination rates within the US.

Skimm'd by Kamini Ramdeen and Niven McCall-Mazza

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