Covid-19 Vaccines

Questions about the COVID-19 virus or the vaccines to combat it? Read here for current research and updates from our authors and educational partners as well as scientists and science writers.

Information about COVID-19

Since December 2019, discussion of COVID-19 has dominated the news and social media. Here are some of the significant moments in the pandemic.

Timeline showing progression of COVID-19. December 2019: first cases of COVID-19 emerge in China; January 21, 2020: First U.S. Case identified, Washington state; January 23, 2020: Chinese city of Wuhan in lockdown; virus emerges in Europe; January 31, 2020: World Health Organization declares a global health emergency; March and April 2020: borders sealed, schools and workplaces closed, events canceled, masking and social distancing started; April 2, 2020: 1 million cases worldwide; June 2020: Infection rates start to go down but then climb again as states attempt to reopen; July 2021: U.S. breaks its record for daily cases and deaths; large scale testing of vaccines begin; September 2020: 1 million deaths worldwide; December 2021: U.S. and U.K. approve emergency use of Pfizer vaccine; virus variants are identified; January 2021: vaccines become available to first responders and individuals 65 and older; 2 millin deaths worldwide; July 2021: delta variant found in 65 countries worldwide; September 2021: 4.5 million deaths worldwide, 5.8 billion does of vaccines given worldwide, 370 million vaccines give in the United States; October 2021: 700,000 people in U.S. dead from COVID-19, which is more than caused by 1918 flu pandemic.

What Is a Virus?

Illustration of the COVID virus. This shows a circular cell studded with protein spikes on the outer coating and RNA on the inside of the cell membrane.

A virus is a small, infectious particle that consists of genetic material (RNA, in the case of the virus that causes COVID-19) and an outer coating that protects the genetic material. Viruses can only live and multiply inside a host, which is a living organism such as a human, animal, or plant. Viruses, like other microbes, have been on Earth for billions of years.

We coexist peacefully with most microbes, and some (like the bacteria in our gut) actually help us. But throughout human history, some pathogens have caused worldwide illness (pandemics).

What Is COVID-19?

COVID-19 is a disease caused by a type of virus called a coronavirus. Viruses cause many diseases, including the common cold, flu, and HIV. Each viral disease is caused by a unique virus with unique features. The COVID-19 virus is covered with proteins in the shape of spikes, like a jeweled crown (corona is Latin for crown).

The COVID-19 virus enters our bodies through our noses and mouths, and then uses its spike proteins to enter cells in our respiratory tracts. Once inside human cells, the virus uses its own genetic material to replicate itself. Eventually, the virus replicates itself sufficiently so that those infected start releasing the virus back out into the world, again through our noses and mouths. That is when we become contagious, spreading the virus to other people.

As our immune systems work to battle the invading virus, we develop the symptoms of the illness, including fever, body aches, and dry cough. Fortunately, most of us have an immune system that is strong enough to win the battle, even though it may take weeks or even months to recover. Unfortunately, many people lose the battle. Worldwide, more than 4.5 million people have died from COVID-19 as of September 2021. As you can see in this information from the Centers for Disease Control and Prevention (CDC), more than 670,000 had died in the United States by the end of September 2021.

The longer viruses are present in our environment, the more likely they are to mutate and form a new strain or variant. The delta variant, now the dominant form of COVID-19 worldwide, is more contagious than the original strain and is more likely to cause severe illness.

Stepwise illustration showing how the virus enters the body. 1. The virus enters the body through the nose and mouth. 2. Once the virus is inside the body, the spike proteins that surround the virus attach to certain proteins in the lungs. 3. The virus releases its RNA into the lung cell, where it copies itself. 4. The replicated RNA allows the virus to create many more copies of itself. 5. The new virus particles are then released by the infected person through their nose and mouth. 6. The original lung cell dies.

How Does COVID-19 Move Through the Environment?

The virus that causes COVID-19 is an airborne virus. This means that it can only travel and reproduce by passing through the respiratory system of mammals like us. The ability for an airborne virus to be infectious depends a lot on the environment. For example, the virus is less concentrated in outdoor and well-ventilated indoor spaces, which reduces its ability to move from one host to another. See this video for an example of how a virus can spread through the air in various environments. (Note that this video recommends keeping a distance of 1 meter, whereas current recommendations call for a distance of 6 feet.)

Should I Worry about Spreading COVID-19?

Before the coronavirus pandemic, did you ever go into work even if you felt a little sniffly? A few days later, one of your co-workers would start showing the same symptoms. A few days after that, a few more people would catch it. “There’s a cold going around,” everyone would say, just part of the world of work. We’d all shrug it off.

Still, back then, even before we were masking and social distancing, we knew that some illnesses were more contagious and more dangerous than others. You might come to work with a cold, but you wouldn’t come to work with the flu, or measles, or chickenpox, because more people would get sick—and some would get seriously sick.

So how contagious is COVID-19 virus, specifically the delta variant that is now the dominant strain? This graphic and information from the Centers for Disease Control and Prevention shows that if you are an unvaccinated person infected with the delta variant, you are likely to spread it to many other people—even if you don’t know you’re infected. The delta variant is a lot more contagious than the flu and almost as contagious as chickenpox.

How many people you spread it to is known as the virus’s reproductive number, or R0. The delta variant’s high R0 is causing the pandemic to spread exponentially. Exponential growth means the virus can spread incredibly quickly.

Illustration showing how an R-naught of 5 multiplies throughout a population. We start with one person, who is likely to spread to 5 other people and end with 6 total. Next we start with 6 people who are likely to spread to 25 other people and end with 30 total. Next we start with 30 people who are likely to spread to 125 other people and end with a total of 150. Next we start with 150 people who are likely to spread to 626 other people and end with 750 total. Finally we start with 750 people who are likely to spread to 3125 other people and end with a total of 3750.

This figure shows how infection spreads using a value of R0 = 5, which means one person is likely to infect another five.

(Newly infected individuals are represented with blue icons. Previously infected individuals are represented with red icons.)

What Is the Delta Variant and Why Is It So Dangerous?

Recent studies show that the delta variant of SARS-CoV-2 (COVID-19) poses a greater threat than earlier versions of the virus. Why? The journal Nature reported findings that indicate that people infected with the delta variant are infectious for longer before they feel sick. So instead of being infectious for part of a day before feeling sick, you’d be infectious for almost two days before feeling sick.

Also, people infected with the delta variant have more virus particles in their bodies than did people who caught the original virus. This increase in viral load makes you more contagious — more likely to spread the virus to others. Scientists calculate that an unvaccinated person with the delta variant is likely to pass it on to five to seven other people, more than twice the contagion of the original strain of the COVID-19 virus.

You’ve probably heard that even people who have been fully vaccinated can catch the delta variant. That is true, but, as reported by Macmillan author and University of Texas at Austin scientist David Hillis, vaccinated people are less likely to get the virus in the first place — and for those who do, they tend to have less virus in their bodies, so they don’t get as sick. They are also less infectious. Writes Hillis, “the vaccines continue to be highly effective at preventing severe illness and hospitalization… the majority of COVID cases, and almost all severe cases of COVID that require hospitalization, are occurring in unvaccinated individuals. Vaccination remains the best protection against COVID-19, and especially against severe cases of the disease…” (Hillis, David, “Immunizations and Booster Shots,”  Mason County Science Corner, Mason County News, August 25, 2021).

Two-part illustration comparing the original COVID-19 strain, where one person is likely to spread the disease to 2 other people, and the mutated delta variant, where one person is likely to spread the disease to 5 to 7 other people.
An illustration of a calendar is shown, indicating that it can take from one to five days for COVID-19 symptoms to arise.

So Far, I'm Good. What Are the Chances I Will Catch COVID-19?

If you have managed to avoid getting sick so far in the pandemic, you may feel that the worst is over. Unfortunately, the current surge indicates that until most of the world’s population is vaccinated, the number of cases will remain high and hospitals will continue to fill with seriously ill patients.

It’s now clear that anyone—vaccinated or unvaccinated—can transmit the disease, although people who are vaccinated are less likely to spread COVID-19 and less likely to become ill. You can reduce your risk by being vaccinated, wearing a mask, social distancing, and avoiding indoor gatherings.

Some areas have been harder hit than others. Are you curious about how widespread COVID-19 is in your area? You can estimate the chance that at least one COVID-19-positive person will be at an event or venue you visit using the COVID-19 Event Risk Assessment Planning Tool. Created by scientists at Georgia Tech, Duke, and Stanford universities, this tool lets you calculate how risky an event may be by adjusting the number of people attending.

You can access statistics about infection rates and deaths by country and see how case numbers are changing daily or monthly at the COVID-19 Dashboard by the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

Information about Vaccines

A vaccine timeline is shown. 1796: first vaccine for smallpox. 1914: pertussis (also known as whooping cough) vaccine; 1926: diptheria vaccine; 1938: tetanus vaccine; 1940s: DTP vaccine, which is a combination of diptheria, tetanus, and pertusis; 1955: polio vaccine; 1960s: measles, mumps, rubella. (Note that these were combined into one vaccine in 1971). 1970s: No naturally occuring cases of smallpox. 1996: chickenpox vaccine; 2006: HPV vaccine; 2007: first study of injecting mRNA directly into human skin, which is the basic science behind the COVID-19 vaccines; 2020: COVID-19 vaccines.

Why Should I Get Vaccinated? I Wear a Mask and Practice Social Distancing.

Graphic illustrating a person wearing a mask; a person washing hands; and two people distancing themselves by 6 feet.

Masking, frequent hand washing, and social distancing are important tools in the fight against COVID-19. For the first year of the pandemic, they were the only protection we had. They are still the only protection for people who do not have access to the vaccine and continue to be important preventive measures. However, the World Health Organization warns that these precautions do not protect you to the same extent as a vaccine.

Instead of Getting Vaccinated, Can't I Just Get Frequent COVID-19 Tests?

A graphic showing icons of a group of people questioning the decision to test or to vaccinate.

Testing is an important tool for controlling the spread of COVID-19, but it does not prevent you from infection. Testing will verify that you were negative for the virus at the time you were tested. It’s a snapshot of your health at that exact moment. It does not guarantee that you will not become infected in a crowded elevator or a coffee shop the moment you leave the testing site. And it does not keep you from spreading the virus to other people.

Think of it this way: if you wear sunscreen at the beach, you will likely avoid a serious sunburn. Or, you could check your shoulders every time you come home from the beach to look for signs of a sunburn. The sunscreen, like a vaccine, is designed to prevent harm. Examining your skin, like a COVID-19 test, only tells you after the fact, and once you’ve gotten a sunburn, all you can do is treat the symptoms and wait for it to go away.

Now imagine a sunburn that is contagious: you scorch your skin at the beach, and then you spread that pain and misery to your family, friends, or coworkers. According to this opinion piece in Scientific American, we don’t talk enough about the effectiveness of vaccines in reducing transmission of the virus: “vaccinating a large majority of Americans throughout the country is our surest bet for returning to normal. Entirely eliminating spread of the virus may be an unreachable goal, but mass vaccination—in the U.S. and around the world—will relegate COVID to the background of our lives.”

Have You Been Waiting for the FDA to Approve the Vaccine?

Icon indicating FDA-approval.

Good news! The Food and Drug Administration approved the Pfizer vaccine on August 23 for individuals 16 years of age and up. Emergency use is still approved for children ages 12 to 15. In addition, the FDA has given the Pfizer vaccine emergency use approval, and the CDC has recommended its use for children ages 5 – 11.

The Pfizer vaccine was the first to receive final FDA approval. Full FDA approval of the Moderna vaccine is expected soon, with approval of the Johnson & Johnson vaccine to follow. For now, both of these vaccines continue to be available under the Emergency Use Authorization.

If I've Already Had COVID-19, Am I Immune to It?

No. Unfortunately, individuals who have had COVID-19 and recovered can get reinfected. In this study reported by the CDC, individuals who were not vaccinated were more than twice as likely to be reinfected compared with those with full vaccination. 

Why Should My Young Children Be Vaccinated?

Young children seem to be as likely as adolescents and adults to be infected with COVID-19 and to spread the virus, even though they seem to be less likely to become seriously ill.

Still, according to the CDC, as of November 1, 2021, 172 children in the United States aged 5 to 11 have died as a result of COVID-19 infection. In addition, there have been more than 8,300 COVID-19-related hospitalizations for children 5 through 11 through September of this year, and about 1.9 million cases of COVID-19 in children in this age group in the Americas since the pandemic began, according to the World Health Organization (WHO).

In studies done among thousands of 5- to 11-year-olds, results showed the Pfizer vaccine had no severe vaccine-related side effects or dangerous allergic reactions. The Pfizer vaccine was found to be more than 90% effective at protecting this age group from contracting symptomatic COVID-19.

Illustration shows a child standing between an adult and a partially filled hypodermic needle with the words "90% effective protection" centered over the needle.

When and Where Can My Children Get Vaccinated?

An FDA approval seal is depicted, along with an adult labeled 18 and older, an adolescent labeled 12 to 17, and a child labeled 5 to 11.
The FDA gave the Pfizer vaccine emergency use approval, and the CDC recommended the Pfizer COVID-19 vaccine for children 5 through 11 on November 2, 2021. This makes about 28 million kids newly eligible for the vaccine. Millions of children’s doses were shipped from the company’s facilities to distribution centers across the country in anticipation of the approval. Vaccines for this age group are now available at the offices of pediatricians and primary care doctors, children’s hospitals, pharmacies, and clinics at schools. Check the CDC website to find COVID-19 vaccines near you.

How Many Shots Will My Children Need?

According to the FDA, children will need to receive two shots of the Pfizer vaccine, and these shots should be administered three weeks apart. The dosage for children 5 to 11 is one third the dose given to adolescents and adults.

A three-column table shows 2 full hypodermic needles under the first column labeled "Pfizer vaccine for adults 18 and older," 2 full hypodermic needles under the second column labeled "Pfizer vaccine for adolescents 12 to 17; and 2 one-third-full hypodermic needles under the third column labeled "Pfizer vaccine for children 5 to 11.

When Will Children Younger than Age 5 Be Eligible for Vaccines?

The companies that produce the three approved vaccines in the United States are studying their vaccines in children 6 months old to 5 years. Those trials have not been completed, although Pfizer has reported it may have data by the end of the year, which will then be submitted to the FDA and CDC for review.

I Hear about Breakthrough Infections: Will I Get Sick Even If I Get Vaccinated?

Breakthrough cases, in which a person who is fully vaccinated becomes infected with COVID-19, do occur, but are relatively rare, according to this article from Scientific American. No vaccine is 100% effective. There will always be some people who get sick in spite of getting vaccinated. This happens with the flu shot every year; many people who get a flu shot later come down with the flu, but vaccinated people are typically not as sick as unvaccinated.

In fact, the COVID-19 vaccine is much more protective than the typical flu shot. The Centers for Disease Control and Prevention provides data that the mRNA vaccines (Moderna and Pfizer) are 90% effective, meaning that a vaccinated person is 10 times less likely to contract COVID-19 than an unvaccinated person.  In contrast, the influenza (flu) vaccine is typically only 40-60% effective. Still, the annual flu vaccine keeps many people from becoming very ill and requiring hospital care, making it an important—if imperfect—weapon against disease.

According to Macmillan Learning author David Hillis, research shows that vaccinated individuals who get breakthrough infections are less likely to become seriously ill, and in fact may be asymptomatic—they may not actually feel sick. Hillis writes, “the severity of any illness is likely to be worse in unvaccinated individuals. So even though the vaccines are not 100% effective at preventing illness, they greatly reduce your chances of getting sick, as well as reducing your chances of severe illness… Eventually, as more of the human population is vaccinated, the virus epidemic is expected to die out. When few people are susceptible to infection, the virus can no longer find new hosts to infect, and the epidemic will be over” (Hillis, David, “Immunizations and Booster Shots,” Mason County Science Corner, Mason County News, August 25, 2021).

Vaccinated individuals are also much less likely to infect others, including vulnerable people who are unvaccinated.

Graph plotting the rate of hospitalization per 100,000 people against time shown from January 30, 2021 through July 3, 2021 for both unvaccinated patients and fully vaccinated patients. The graph shows a very slight increase from 0 to 10 for fully vaccinated patients. The graph shows a more dramatic rise from 0 to 420 for unvaccinated patients.

What Are the Vaccine's Side Effects?

Many people are concerned about the side effects of the COVID-19 vaccine. All vaccines, and indeed all medications, carry some risk of side effects. It’s important to be aware of those risks, according to Jennifer Punt, a Macmillan Learning author and University of Pennsylvania immunologist. In this video Dr. Punt speaks plainly about the risk of side effects from the COVID-19 vaccine versus becoming infected with the virus.

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Side effects may include pain, redness, and swelling around the area where you received the shot (typically, your arm), and fatigue, headache, muscle pain, fever, chills, and nausea. For most people, side effects only last a day or two.  Side effects are typically worse after the second dose. The discomfort is a sign that your body is developing an immune response that will protect you if you are exposed to the virus.

For some people, side effects may be greater. In this video, Macmillan Learning author and Pomona College professor of biology Sharon Stranford describes the typical side effects of the COVID-19 vaccines. If you have concerns about potential side effects from the vaccine, talk to a qualified medical professional before you receive your first jab.

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What about the Rumors that the Vaccines Are Dangerous?

There are a lot of rumors about the COVID-19 vaccines. Some of this information is inaccurate and dangerous, but may sound convincing. According to Macmillan Learning author and University of Pennsylvania professor of immunology Jennifer Punt, we are all susceptible to misinformation. 

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Let’s look at the rumors one at a time.

The vaccine will give me COVID-19. This is a common concern based on the fact that in the past, some vaccines contained live virus particles. None of the COVID-19 vaccines used in the United States contain live COVID virus, and so they cannot give you the disease. Instead, as described in this article from the CDC, vaccines prepare our immune systems to fight off the disease if we are exposed. That is why some people feel ill for a day or two after receiving the vaccine.

The vaccine can affect fertility, pregnancy, or sexual function. According to Scientific American, “studies so far have not linked the vaccines with problems related to pregnancy, menstrual cycles, erectile performance or sperm quality. The evidence does show that COVID-19 can involve problems in all of these areas.” The CDC recommends that pregnant and breastfeeding women be vaccinated against the virus. Because of changes to the immune system during pregnancy, pregnant unvaccinated women are five times more likely to contract the virus than an unvaccinated woman who is not pregnant. What’s more, having the virus during pregnancy is more likely to make you seriously ill.

The vaccine causes dangerous blood clots. There is a very slight risk of developing blood clots after receiving a COVID-19 vaccine, studies show. However, this study cited in the British Medical Journal found that the risk of blood clotting events after infection with COVID-19 is much higher than the risk posed by vaccination.

Beware of false or misleading information. This article in the journal Nature lists eight ways to spot misinformation.

The organization #ScienceUpFirst is a Canadian-based group of independent scientists and health care professionals focused on sharing the best available science with the public. You can follow them on Twitter, Instagram, Facebook, and TikTok: @ScienceUpFirst. They also post in French: @LaScienceAbord.

Want to learn more, or have questions about things you’ve heard about or read about? The World Health Organization (WHO) has a comprehensive list of myths about COVID-19 and vaccines.

What Is a Booster Shot, and How Does It Differ from the Initial Vaccine?

When you get a vaccine, you are teaching your immune system what a particular invading bacteria or virus looks like. If the bacteria or virus appears again in the future, your body can keep you healthy by quickly recognizing and destroying it. Your immune system protects you by developing a defense that includes antibodies (blood proteins produced by your body to counteract germs) and important immune cells. To help your body produce an effective immune response, many vaccines need multiple doses, usually spaced out over a few weeks or months. In some cases this is enough to protect you for a lifetime. More commonly, though, your body needs occasional reminders to keep up its defenses, and these reminders come in the form of booster shots. You’ve probably received a booster shot of a vaccine before–for example, booster shots to protect against tetanus are recommended every 10 years, during pregnancy, and when certain injuries occur.

The booster shots of both the Pfizer and Johnson & Johnson vaccines are the same as the shots used for initial vaccination: the same substance and the same dose. For the Moderna vaccine, the substance is the same but a smaller dose is used in the booster shot than in the initial series.

Table is shown comparing vaccine types by the number and dose of second shots and boosters. The first row shows the Pfizer vaccine, and illustrates one full hypodermic needle under the column labeled "First Shot"; one full hypodermic needle under the column labeled "Second Shot"; and one full hypodermic needle under the column labeled "Booster." The second row shows the Moderna vaccine, and illustrates one full hypodermic needle under the column labeled "First Shot"; one full hypodermic needle shown under the column labeled "Second Shot"; and one partially filled hypodermic needed under the column labeled "Booster." The third row shows the Johnson and Johnson vaccine, and illustrations one full hypodermic needle under the column labaled "First Shot"; a null symbol under the column labeled "Second Shot"; and a full hypodermic needle shown under the column labeled "Booster."

Will I Need a Booster Shot?

Currently, you do not need to get a booster shot to be considered fully vaccinated. However, the CDC has recommended booster shots for several different groups of people based on age and other risk factors.

  • For people who received two doses of either the Pfizer or Moderna vaccines, the CDC recommends booster shots for all adults age 65 or older as well as some groups of adults over the age of 18 who have specific risk factors. These booster shots should be administered at least six months after the second shot.
  • For people who received one dose of the Johnson & Johnson vaccine, anyone over the age of 18 is recommended to get a booster shot at least two months after their initial vaccination.
Two graphics are shown. The first graphic illustrates a person and two hypodermic needles with the label "Fully vaccinated" beneath it; to the right is a plus sign and a third hypodermic needle with the label "Booster" beneath it. To the right of the art are multiple question marks.

My Immune System Is Compromised; What’s Recommended for Me?

For some individuals with compromised immune systems, vaccination guidelines may include additional doses. For more information, you can consult the CDC’s website or contact a trusted healthcare provider to discuss what’s best for you.

I'm Eligible for a Booster Shot. Does It Matter Which One I Get?

The CDC currently allows the Pfizer, Moderna, and Johnson & Johnson vaccines to be used as boosters regardless of which vaccine was initially administered. In some cases, it may make sense to boost with a different brand of vaccine–for example if you have concerns about specific reactions or side effects, or if a different brand of vaccine is the only one available. Studies have suggested that in some cases there may be benefits to having different brands of vaccines for the initial and booster vaccinations; however, more studies are needed to know if one series of vaccinations is better than another. For additional information about which booster shots may be appropriate, you can use an interactive quiz developed by National Public Radio to determine which booster shots are available to you based on your specific circumstances.

What Should I Expect When I Return to Work Onsite?

Many people are anxious about returning to a worksite where they will be in close quarters with their colleagues. After more than a year of isolation, returning to work will require a psychological and physical adjustment, as noted in this Scientific American article. The safest way to return to a shared work space is to:

  • Get vaccinated. Doing so protects you and the people around you, some of whom may not be able to receive the vaccine for medical reasons.
  • Wear a mask when you’re indoors.
  • Practice social distancing when possible. Spread out in meetings, and don’t crowd the elevators. Give everyone some space.
  • Wash your hands frequently.
  • Observe testing requirements.
  • If you’re not feeling well or you or a family member is exposed to the virus, stay home and arrange for a COVID-19 test. Don’t come back to work until you are cleared to do so by a medical professional and by your employer.

For more information about how your employer or campus is addressing a return, please contact your Human Resources or Campus Safety and Administration departments.

Illustration shows the options to vaccinate, mask, distance, and hand wash.