Four coronavirus vaccine candidates have advanced to late-stage clinical trials in the U.S., and another should begin late-stage testing soon. One main purpose of these clinical studies is to determine whether the candidates are effective at protecting people from coronavirus infection, and if so, just how effective. Another key goal is to demonstrate whether or not they're safe.

It's the latter point that has some Americans worried right now. AstraZeneca's late-stage study of coronavirus vaccine candidate AZD1222 remains paused in the U.S. because of a potential safety concern. A news story broke Monday evening that Johnson & Johnson has also paused its phase 3 study of JNJ-78436735 because of a possible safety issue. In addition, reports have emerged from the clinical trials of other leading vaccine candidates about adverse reactions in some participants.

Is this news really cause for alarm? Here are three key things you need to know about coronavirus vaccine side effects. 

Physician holding a vaccine shot next to a person's arm

Image source: Getty Images.

1. Some side effects have occurred with all of the leading vaccine candidates.

All four of the coronavirus vaccine candidates in phase 3 clinical trials in the U.S. have produced at least some side effects in some study participants. Here's a quick rundown on what has been reported so far:

Company

Vaccine Candidate 

Reported Side Effects

AstraZeneca's (NASDAQ:AZN) AZD1222
  • Injection site pain and tenderness
  • Mild-to-moderate headache
  • Fatigue
  • Chills
  • Feverishness
  • Malaise
  • Muscle ache
  • Unexplained illness in one participant 
Johnson & Johnson (NYSE:JNJ)  JNJ-78436735
  • Injection site pain
  • Fever
  • Fatigue
  • Headache
  • Myalgia (muscle pain)
  • Unexplained illness in one participant
Moderna (NASDAQ:MRNA) mRNA-1273
  • Injection site pain
  • Headache
  • Fatigue
  • Myalgia (muscle pain)
  • Chills
  • Fever
Pfizer (NYSE:PFE) / BioNTech (NASDAQ:BNTX) BNT162b2
  • Injection site pain
  • Fatigue
  • Fever
  • Headache
  • Chills
  • Muscle pain
  • Joint pain

Data sources: Company press releases and clinical trial updates.

The most concerning of those are the unexplained illnesses that occurred with participants in AstraZeneca's late-stage study of AZD1222 in the U.K. and J&J's late-stage study of JNJ-78436735. Although the U.S. trial of AZD1222 remains paused, regulators in other countries, including the U.K., Japan, and South Africa, have allowed AstraZeneca to resume late-stage studies. There aren't enough details available yet related to J&J's clinical trial pause to know how long it might last.

All of the other side effects reported for these coronavirus vaccine candidates in earlier clinical trials resolved relatively quickly. This also appears to be the case in the late-stage studies. For example, CNBC recently interviewed some participants in phase 3 clinical trials evaluating Moderna's and Pfizer's coronavirus vaccine candidates. These participants reported experiencing side effects including "high fever, body aches, bad headaches, and exhaustion." However, all of them said those issues went away after no more than a day.

2. A chance of side effects is normal with any vaccine.

It's important to understand that every vaccine on the market, regardless of what it's for, will probably cause some side effects, some of the time. Most of the side effects reported so far with the leading coronavirus vaccine candidates also occur with several of the top vaccines that have already been approved by the FDA for other diseases.

For example, common side effects of Pfizer's pneumococcal vaccine Prevnar 13 include injection site pain, fatigue, headache, muscle pain, joint pain, fever, and chills. Other common side effects for Prevnar 13 are limitation of arm movement, decreased appetite, vomiting, and rash. It's a similar story for GlaxoSmithKline's shingles vaccine Shingrix. Common side effects include injection site pain and swelling, muscle pain, tiredness, headache, shivering, fever, and upset stomach.

Remember that all vaccines spur the body's immune system to fight against a perceived invader. Several of the common side effects are natural results of this immune response. 

Also, it's important to understand that there's a big difference between what you might mean by "common" and what a pharmaceutical company does. When a drugmaker says "common side effects include...", each symptom it lists has been found to occur in somewhere between 1% and 10% of the people who receive the medication.

3. No vaccine will win FDA emergency use authorization if its side effects are too worrisome.

The Food and Drug Administration recently released the guidelines it will use for its emergency use authorization (EUA) process for coronavirus vaccines. The agency clearly stated, "Issuance of an EUA requires a determination that the known and potential benefits of the vaccine outweigh the known and potential risks."

Vaccine makers must provide all safety data for their vaccine candidates, including the late-stage data plus all safety data from earlier clinical studies. In addition, the FDA is asking that companies monitor participants for at least two months after full dosing (which for most of these candidates means after they have received the second of two doses).

The agency will also include an advisory committee in its EUA review process, members of which include infectious disease experts from academia, government agencies, and the private sector. While the FDA doesn't have to abide by the advisory committee's recommendations, it seems unlikely that a coronavirus vaccine will win an EUA if a majority of the committee recommends against it, given the political dynamics at work.

Key vaccine data is coming soon

The next few months are likely to be quite exciting for the healthcare sector. Pfizer and BioNTech expect to announce preliminary late-stage results later this month, while Moderna anticipates that it could formally open its request for an EUA for mRNA-1273 by late November. You probably won't have to wait much longer to know more about how safe and effective the leading coronavirus vaccines are.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.