On Friday, U.S. regulators authorized Moderna Inc’s COVID-19 vaccine for emergency use, a week after granting the first U.S. authorization to Pfizer Inc and BioNTech SE’s coronavirus shot.

Here is a comparison of the two vaccines and what to expect as both become available:

How are the vaccines alike?

Both these vaccines use messenger RNA (mRNA) technology, which contains instructions for human cells to make proteins that mimic the coronavirus. The instructions drive the immune system into action, turning the body into a virus-zapping vaccine factory. The vaccines do not contain the real virus.

Both the Pfizer/BioNTech and Moderna vaccines work by targetting the crown-like spikes on the coronavirus’s surface that it uses to break into healthy human cells. These spikes or pointed tips also give the family of viruses their distinctive name.

The vaccines appear to be similarly effective. Pfizer and BioNTech’s vaccine was around 95% effective at preventing disease symptoms in its late-stage trial, while Moderna’s vaccine was about 94% effective.

Patients need to take both these vaccines in two doses; Pfizer’s 21 days apart and Moderna’s 28 days apart. Very few participants who received the vaccine in either trial got sick with COVID-19, and almost none got seriously ill.

Data the companies have submitted to the Food and Drug Administration suggest that they start offering partial protection against COVID-19 around two weeks after recipients receive their first dose.

How are they distinct?

Cold vs. ultra-cold: The principal difference between the two vaccines lies in the temperature needed to store them. A suitable temperature is required to keep them stable long-term.

Pfizer’s vaccine needs ultra-cold temperatures of minus 70 degrees Celsius (-94 Fahrenheit) for its storage. It requires five days of refrigeration once it becomes thawed. Also, it needs a particular shipping container packed with dry ice to keep it at the proper temperature.

Moderna’s vaccine needs standard freezer temperatures of -20 Celsius (-4 Fahrenheit) for up to six months for its storage. It needs refrigeration for up to 30 days after it’s thawed.

SIDE EFFECTS AND SAFETY

The two large-scale clinical trials of the vaccines did not show any long-term severe side effects. Although the vaccines slightly differ in the short-term side effect profiles.

Although there is no head-to-head comparison data of the two vaccines available, Moderna’s vaccine seems to be associated with slightly more severe cases of headache, fatigue, and fever in a day or two after the second shot, particularly in people younger than 65.

Although not seen in its clinical trial, Pfizer’s vaccine finds its association with a few cases of severe allergic reactions in the U.K. and the United States. At least two healthcare workers in Britain and two in Alaska have reported severe allergic reactions soon after receiving the Pfizer/BioNTech vaccine.

Britain’s medical regulator has said anyone with a history of anaphylaxis or severe allergic reactions to medicine or food should not get the Pfizer/BioNTech COVID-19 vaccine.

But as per the Food and Drug Administration (U.S.), it should be safe for most Americans with allergies to receive the vaccine.

The pre-clinical and clinical trials test the safety of each coronavirus vaccine. The most common approved side effects of coronavirus vaccines are:

• Tiredness or fatigue
• Headache
• Muscle pain
• Chills
• Pain at the injection site
• Joint pain
• Fever
• Swollen lymph nodes (lymphadenopathy)
• Swelling or redness at the injection site
• Nausea
• Feeling unwell

These side effects subside after 1 or 2 days. Most people have mild or moderate symptoms, while for some people, symptoms may be more severe.

People with a known history of a severe allergic reaction to any coronavirus vaccine component should not receive the vaccine, the FDA recommends.

The agency also says people who have a severe allergic reaction to the first dose of a two-dose vaccine should not receive the second dose.

U.S. nurse faints after receiving Pfizer-BioNTech COVID-19 vaccine shot.

At a press conference, Tiffany Dover, a head nurse at a Chattanooga hospital in Tennessee, caused a scare as she fainted after receiving the Pfizer-BioNTech COVID-19 vaccine shot.

Speaking at a press conference at CHI Memorial Hospital in Chattanooga, Tennessee, she said, “I and all of my staff are thrilled to get the vaccine. We work in the COVID unit, so, therefore, my team will be getting the first prospects of getting the vaccine.”

Amidst her interaction with the press, Dover suddenly started feeling uneasy and said, “Sorry, I’m feeling dizzy.” Medical experts came to assist the nurse after she fainted. The nurse has now recovered and is feeling better.

US approves Moderna vaccine for Covid-19

On Friday, the U.S. added a second Covid-19 vaccine to its arsenal. It will boost the efforts of dealing with the coronavirus outbreak in the nation that is regularly claiming more than 3,000 lives a day.

Much-needed doses will arrive on Monday after the Food and Drug Administration’s authorization for the emergency rollout of the vaccine developed by Moderna Inc. and the National Institutes of Health.

The move signifies the world’s first authorization for Moderna’s shots. The vaccine resembles Pfizer Inc. and Germany’s BioNTech’s vaccine, which is already in the process of giving to millions of health care workers and nursing home residents as the biggest vaccination drive in U.S. history starts to ramp up.

WILL MASKS STILL BE NEEDED?

More data is required to understand precisely how effective either vaccine is at preventing virus transmission. While both are highly effective at keeping symptoms and severe illness at bay, we do not know if they prevent someone from becoming infected. Until that is known, experts say, people will need masks to ensure that vaccinated ones do not spread the virus.

Will Americans get to choose?

Currently, with a short supply of vaccines, few people will enjoy the luxury of expressing a preference of which they receive. The US Centers for Disease Control and Prevention has recommended the first doses to healthcare workers and residents in long-term care facilities, and most states plan to do that.