COVID-19 Vaccine: Types, Side Effects & Who Should Get Vaccinated

What Is a COVID-19 Vaccine and How Does It Work?

COVID-19 vaccines work by teaching your immune system to recognize and fight the SARS-CoV-2 virus without causing infection. They contain instructions for making spike proteins (mRNA vaccines), harmless modified viruses carrying genetic code (viral vector), purified spike proteins (protein subunit), or inactivated virus particles. Once vaccinated, your body produces antibodies and memory cells that provide protection against future infection.

COVID-19 vaccines represent one of the most significant achievements in modern medicine. Developed at unprecedented speed through global scientific collaboration, these vaccines have proven instrumental in reducing the severity of the pandemic. The vaccines train your immune system to recognize the distinctive spike protein found on the surface of the SARS-CoV-2 virus, which the virus uses to enter human cells.

When you receive a COVID-19 vaccine, your body responds by producing several types of immune cells. B cells create antibodies that can neutralize the virus, preventing it from entering cells. T cells identify and destroy cells that have been infected. Perhaps most importantly, your immune system creates memory cells that remain in your body long after vaccination, ready to mount a rapid response if you encounter the actual virus.

The development of COVID-19 vaccines built upon decades of prior research into coronavirus biology and vaccine technology. Scientists had previously studied related coronaviruses, including those causing SARS and MERS, which accelerated the development process. The genetic sequence of SARS-CoV-2 was shared globally in January 2020, allowing researchers worldwide to immediately begin vaccine development.

How mRNA Vaccines Work

mRNA vaccines, such as those from Pfizer-BioNTech (Comirnaty) and Moderna (Spikevax), use messenger RNA to instruct your cells to produce a harmless piece of the spike protein. This protein triggers an immune response without any risk of infection because the vaccine contains no live virus. The mRNA is quickly broken down by your body and does not enter the cell nucleus or interact with your DNA.

The lipid nanoparticle delivery system protects the fragile mRNA and helps it enter cells. Once inside, the mRNA is read by ribosomes in the cytoplasm, which produce the spike protein. This protein is then displayed on the cell surface, where immune cells recognize it as foreign and mount a response. The entire process is temporary—the mRNA degrades within days, but the immune memory persists for months to years.

How Viral Vector Vaccines Work

Viral vector vaccines use a modified version of a different virus (the vector) to deliver genetic instructions to your cells. The AstraZeneca vaccine uses a chimpanzee adenovirus, while the Johnson & Johnson vaccine uses a human adenovirus. These vectors have been genetically modified so they cannot replicate in your body or cause illness.

Once injected, the viral vector enters cells and releases its genetic payload, which instructs cells to produce the spike protein. This approach has been used successfully in other vaccines and offers the advantage of stability at standard refrigerator temperatures, making distribution easier in resource-limited settings.

How Protein Subunit Vaccines Work

Protein subunit vaccines, such as Novavax (Nuvaxovid), contain purified spike proteins rather than genetic instructions. These proteins are produced in laboratories using insect cells or yeast and are combined with an adjuvant—a substance that enhances the immune response. This traditional vaccine approach has been used for decades in vaccines against hepatitis B and pertussis.

How Inactivated Virus Vaccines Work

Inactivated virus vaccines, including Sinovac (CoronaVac) and Sinopharm, contain SARS-CoV-2 virus particles that have been killed using chemicals or heat. While these viruses cannot cause infection, they retain their structure and can stimulate an immune response. This is one of the oldest and most established vaccine technologies.

What Are the Different Types of COVID-19 Vaccines?

There are four main types of COVID-19 vaccines: mRNA vaccines (Pfizer-BioNTech, Moderna) with approximately 95% efficacy; viral vector vaccines (AstraZeneca, Johnson & Johnson) with 60-85% efficacy; protein subunit vaccines (Novavax) with approximately 90% efficacy; and inactivated virus vaccines (Sinovac, Sinopharm) with 50-80% efficacy. All types provide strong protection against severe disease and death.

Understanding the different vaccine technologies can help you make informed decisions about vaccination. Each type has distinct characteristics, manufacturing processes, storage requirements, and efficacy profiles. Importantly, all authorized vaccines have been rigorously tested and shown to be safe and effective at preventing severe COVID-19.

It's important to note that efficacy numbers can vary depending on which variant is circulating and how much time has passed since vaccination. Real-world effectiveness studies have consistently shown that all vaccine types provide excellent protection against hospitalization and death, even when effectiveness against infection decreases.

Updated Vaccines for Current Variants

As SARS-CoV-2 evolves, vaccine manufacturers update their formulations to target currently circulating variants. These updated vaccines are sometimes called "bivalent" when they target two strains, or they may be monovalent formulations targeting the dominant circulating variant. Health authorities regularly review circulating variants and make recommendations about which vaccine formulations should be used.

The process of updating vaccines is similar to the annual updates made to influenza vaccines. Regulatory agencies have streamlined the approval process for variant-updated COVID-19 vaccines, allowing faster deployment while maintaining rigorous safety standards.

How Effective Are COVID-19 Vaccines?

COVID-19 vaccines are highly effective at preventing severe disease, hospitalization, and death. Initial clinical trials showed 90-95% efficacy against symptomatic infection. While effectiveness against infection may decrease over time and with new variants, protection against severe outcomes remains strong at 80-90%. Booster doses restore and enhance protection.

Vaccine effectiveness must be understood in context. Clinical trials measure efficacy under controlled conditions, while real-world effectiveness can vary based on factors including circulating variants, time since vaccination, and individual immune responses. Despite these variations, the core benefit of COVID-19 vaccines—protection against severe illness—has remained remarkably consistent.

Large-scale studies involving millions of people have confirmed the vaccines' effectiveness. Research from multiple countries has shown that vaccinated individuals are significantly less likely to require hospitalization, intensive care, or mechanical ventilation compared to unvaccinated individuals. The risk reduction is particularly pronounced for the most serious outcomes.

The concept of "waning immunity" refers to the natural decrease in antibody levels over time following vaccination. However, protection against severe disease wanes more slowly than protection against infection because it relies not just on antibodies but also on memory B cells and T cells that persist longer. This is why someone who is vaccinated may still become infected but is much less likely to become seriously ill.

Protection Across Age Groups

Studies have shown that vaccine effectiveness varies somewhat by age group. Older adults and immunocompromised individuals may have a less robust initial immune response, which is why these groups are often prioritized for booster doses. However, vaccines still provide substantial protection for these vulnerable populations, who are at highest risk of severe outcomes from COVID-19.

Children generally mount strong immune responses to vaccination. Pediatric vaccine doses are adjusted based on age, with younger children receiving smaller doses. Studies have confirmed that vaccines are safe and effective in children as young as 6 months, though the disease risk and vaccine recommendations may differ from those for adults.

Hybrid Immunity

Research has shown that individuals who have both been vaccinated and recovered from COVID-19 infection develop particularly robust immunity, sometimes called "hybrid immunity." This combination appears to provide broader and more durable protection than either vaccination or infection alone. However, health authorities do not recommend intentionally seeking infection, as the risks of COVID-19 disease outweigh any potential immunological benefits.

What Are the Common Side Effects of COVID-19 Vaccines?

Common side effects include injection site pain (70-90%), fatigue (50-70%), headache (40-65%), muscle pain (30-60%), chills (20-40%), fever (10-40%), and nausea (10-20%). These typically appear within 24 hours and resolve within 1-3 days. Side effects are signs that your immune system is responding. Serious side effects are extremely rare.

Experiencing side effects after vaccination is normal and indicates that your immune system is building protection. The side effects of COVID-19 vaccines are similar to those of other vaccines and are generally mild to moderate in intensity. Most people can manage symptoms with rest, hydration, and over-the-counter pain relievers if needed.

The immune response triggered by vaccination involves the activation of various inflammatory pathways, which can cause systemic symptoms like fever, fatigue, and muscle aches. These effects are temporary and resolve as the immune response stabilizes. The intensity of side effects does not necessarily indicate the strength of the immune response—people who experience minimal side effects may still develop excellent protection.

Differences Between First and Subsequent Doses

Many people notice that side effects differ between vaccine doses. For mRNA vaccines, side effects are often more pronounced after the second dose compared to the first. This is because your immune system has already been primed by the first dose and responds more vigorously to the second. Booster doses may also produce noticeable side effects, though many people report they are similar to or milder than those experienced with earlier doses.

When to Seek Medical Attention

While most side effects are minor and resolve on their own, certain symptoms warrant medical attention. Contact a healthcare provider if you experience severe headache that doesn't respond to pain medication, difficulty breathing, persistent chest pain, severe abdominal pain, or leg pain or swelling. These could indicate rare but serious complications that require evaluation.

What Are the Rare Serious Side Effects?

Rare serious side effects include anaphylaxis (approximately 2-5 per million doses), myocarditis/pericarditis (particularly in young males after mRNA vaccines, approximately 1-4 per 100,000), and thrombosis with thrombocytopenia syndrome (TTS) with viral vector vaccines (approximately 1 per 50,000-100,000). These conditions are treatable when identified early. The benefits of vaccination far outweigh these small risks.

Extensive safety monitoring of billions of vaccine doses has identified several rare but serious side effects. Understanding these risks in context is important—while any serious side effect is concerning, the absolute risk remains very low, and COVID-19 itself poses significantly greater risks of serious complications including myocarditis, blood clots, and death.

Anaphylaxis

Anaphylaxis is a severe allergic reaction that can occur with any vaccine or injectable medication. It typically occurs within 15-30 minutes of vaccination, which is why vaccination sites require observation periods after injection. Symptoms include difficulty breathing, swelling of the face or throat, rapid heartbeat, dizziness, and widespread rash. Treatment with epinephrine is highly effective when administered promptly.

The rate of anaphylaxis following COVID-19 vaccination is estimated at 2-5 cases per million doses—slightly higher than some other vaccines but still extremely rare. People with a history of severe allergic reactions to vaccine components should discuss vaccination with their healthcare provider.

Myocarditis and Pericarditis

Myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the heart lining) have been associated primarily with mRNA vaccines, particularly in adolescent and young adult males after the second dose. The risk is estimated at approximately 1-4 cases per 100,000 doses in the highest-risk group.

Importantly, most cases are mild and resolve with rest and anti-inflammatory medication. Studies have shown that vaccine-associated myocarditis is generally less severe than myocarditis caused by COVID-19 infection, which occurs at significantly higher rates. The overall benefit-risk assessment continues to favor vaccination, though some countries have adjusted recommendations for specific age groups.

Thrombosis with Thrombocytopenia Syndrome (TTS)

TTS is a rare condition involving blood clots in unusual locations combined with low platelet counts. It has been associated primarily with adenoviral vector vaccines (AstraZeneca, Johnson & Johnson). The mechanism involves an immune response that activates platelets, similar to a condition called heparin-induced thrombocytopenia.

The risk is estimated at approximately 1 per 50,000-100,000 doses and appears to be higher in younger individuals. Recognition of this side effect led many countries to restrict or discontinue use of viral vector vaccines for certain age groups and to prefer mRNA or protein subunit vaccines where available.

Who Should Get Vaccinated Against COVID-19?

COVID-19 vaccination is recommended for almost everyone aged 6 months and older. Priority groups include adults 65+, immunocompromised individuals, healthcare workers, and those with underlying conditions (diabetes, heart disease, lung disease, obesity, cancer). Vaccination is safe and recommended during pregnancy and breastfeeding. Very few absolute contraindications exist.

Public health recommendations for COVID-19 vaccination have evolved as more data has become available and as the pandemic has transitioned to an endemic phase. While initial vaccine rollouts prioritized highest-risk groups due to limited supply, vaccines are now widely available in most countries for anyone who wants protection.

The fundamental principle guiding vaccination recommendations is that the benefits of vaccination outweigh the risks for nearly everyone. This calculus takes into account both individual protection and community-level benefits. While no vaccine is 100% effective, widespread vaccination significantly reduces virus transmission and protects vulnerable individuals who may not mount strong immune responses.

High-Priority Groups for Vaccination and Boosters

• Adults aged 65 and older: Higher risk of severe illness and death from COVID-19
• Immunocompromised individuals: May need additional primary doses and more frequent boosters
• Healthcare workers: Higher exposure risk and importance of protecting patients
• People with underlying health conditions: Including diabetes, heart disease, chronic lung disease, kidney disease, obesity, and cancer
• Pregnant individuals: COVID-19 poses increased risks during pregnancy
• Residents of long-term care facilities: High-risk environments with vulnerable populations

Vaccination During Pregnancy and Breastfeeding

COVID-19 vaccination is strongly recommended during pregnancy. Pregnant individuals who contract COVID-19 are at increased risk of severe illness, preterm birth, and other pregnancy complications. Studies involving tens of thousands of pregnant individuals have found no increased risk of miscarriage, birth defects, or other adverse pregnancy outcomes following vaccination.

Vaccination during pregnancy provides an additional benefit: antibodies cross the placenta and are present in breast milk, providing some passive immunity to newborns. This is particularly valuable since vaccines are not yet approved for infants under 6 months of age.

Who Should NOT Get Vaccinated?

Very few people have absolute contraindications to COVID-19 vaccination. The main contraindications include:

• Severe allergic reaction (anaphylaxis) to a previous dose: of the same vaccine type
• Known allergy to vaccine components: such as polyethylene glycol (PEG) for mRNA vaccines

People who have experienced milder allergic reactions, have other allergies (including to foods, medications, or other vaccines), or are acutely ill should consult their healthcare provider but can usually be safely vaccinated with appropriate precautions.

Do I Need a COVID-19 Booster Dose?

Booster doses are currently recommended for adults 65+, immunocompromised individuals, healthcare workers, and people with high-risk conditions. Updated vaccines targeting current variants are preferred when available. Healthy adults under 65 should consult current guidelines, as recommendations vary by country and may change. Boosters restore waning immunity and improve protection against severe disease.

The need for booster doses reflects the natural waning of immunity over time. Unlike some vaccines that provide lifelong protection, COVID-19 vaccines may require periodic boosters to maintain optimal protection—similar to the annual influenza vaccine. The emergence of new variants that partially evade existing immunity further supports the value of updated booster doses.

Booster recommendations have evolved significantly and vary between countries. Some health authorities recommend regular boosters for high-risk groups while advising healthy younger adults that additional doses are optional. Others maintain broader booster recommendations. It's important to consult current guidelines from your local health authority.

Who Should Prioritize Booster Doses?

• Adults 65 years and older: Higher risk of severe outcomes and may have weaker initial immune response
• Immunocompromised individuals: Including organ transplant recipients, cancer patients on chemotherapy, people with HIV, and those taking immunosuppressive medications
• Healthcare workers and front-line workers: Higher exposure risk
• Residents of long-term care facilities: Vulnerable populations in high-risk settings
• People with multiple underlying conditions: Especially those with conditions affecting multiple organ systems

Timing of Booster Doses

The optimal timing for booster doses depends on several factors including age, immune status, and time since previous vaccination or infection. Most guidelines recommend at least 2-4 months between doses, though longer intervals may provide a better immune response. Health authorities update timing recommendations as new data becomes available.

If you have recently recovered from COVID-19, you may be advised to wait before receiving a booster dose, as natural infection provides temporary protection. The length of this waiting period varies by guideline and individual circumstances.

How Do I Get a COVID-19 Vaccine?

COVID-19 vaccines are available at healthcare facilities, pharmacies, community vaccination sites, and sometimes workplaces. Check your local health authority's website for locations and eligibility. Most places offer walk-in appointments, though scheduling may be required. Bring identification and any previous vaccination records. The injection takes seconds, with a 15-minute observation period afterward.

Access to COVID-19 vaccines has improved significantly since the initial rollout. In most countries, vaccines are widely available at various locations, often free of charge. The specific process for obtaining a vaccine varies by country and region, but general steps are similar.

Steps to Get Vaccinated

1. Check eligibility and find locations: Visit your local health department or national health authority website to find vaccination sites near you
2. Schedule an appointment: Some locations require appointments while others offer walk-in service. Online scheduling is often available
3. Bring required documents: Identification and any previous vaccination records or cards
4. Complete pre-vaccination screening: Answer questions about allergies, current illness, and vaccination history
5. Receive the vaccine: The injection is given in the upper arm and takes only a few seconds
6. Wait for observation: Remain at the vaccination site for 15-30 minutes for monitoring
7. Get your vaccination record: Receive documentation of your vaccination for personal records and potential future requirements

What to Expect at Your Appointment

Vaccination appointments are typically quick and straightforward. You'll be asked screening questions to ensure the vaccine is appropriate for you. The injection itself takes only a few seconds and is administered in the deltoid muscle of the upper arm. You may feel a brief pinch or pressure.

After vaccination, you'll be asked to wait 15-30 minutes for observation. This is a precautionary measure to ensure immediate medical care is available in the unlikely event of an allergic reaction. During this time, you can sit comfortably and are free to use your phone, read, or simply rest.

What Are Common Myths About COVID-19 Vaccines?

Common myths include that vaccines alter DNA (false—mRNA doesn't enter the nucleus), cause infertility (no evidence after billions of doses), were developed too quickly to be safe (built on decades of research with rigorous testing), contain microchips (technologically impossible), or provide better immunity than natural infection (both provide protection, but vaccination is far safer).

Misinformation about COVID-19 vaccines has spread widely during the pandemic. Understanding the facts behind common myths can help you make informed decisions and discuss vaccination with others who may have concerns.

Myth: COVID-19 Vaccines Alter Your DNA

Fact: mRNA vaccines do not alter your DNA. The mRNA from vaccines remains in the cytoplasm of cells and never enters the nucleus where DNA is stored. mRNA is naturally fragile and breaks down within days. Additionally, the human body does not have the enzymes needed to convert mRNA into DNA. This myth likely arose from confusion about how genetic information flows in cells, but decades of molecular biology research confirm that mRNA cannot become part of your genetic code.

Myth: COVID-19 Vaccines Cause Infertility

Fact: There is no evidence that COVID-19 vaccines affect fertility in men or women. This myth originated from a false claim about similarities between the spike protein and a protein involved in placenta development, but the proteins are not similar enough to cause cross-reaction. Studies of vaccinated individuals show no impact on fertility, sperm quality, or pregnancy outcomes. Major fertility organizations worldwide recommend vaccination for those trying to conceive.

Myth: Vaccines Were Developed Too Quickly to Be Safe

Fact: While COVID-19 vaccines were developed faster than any previous vaccines, this was due to unprecedented resources, global collaboration, and process efficiencies—not shortcuts in safety testing. The technologies used (particularly mRNA) had been researched for decades. Clinical trials enrolled tens of thousands of participants and followed standard protocols. Ongoing surveillance of billions of doses has confirmed the vaccines' safety profile.

Myth: Natural Immunity Is Better Than Vaccine Immunity

Fact: While natural infection does provide immunity, it comes with significant risks including severe illness, long-term complications (long COVID), and death. Vaccination provides protection without these risks. Studies have shown that vaccination after recovery (hybrid immunity) provides the strongest and most durable protection. Health authorities consistently recommend vaccination regardless of prior infection status.

What Are the Long-Term Effects of COVID-19 Vaccines?

Extensive follow-up data shows no concerning long-term effects from COVID-19 vaccines. Vaccine side effects typically appear within days to weeks—not months or years. The vaccine components are processed and eliminated by the body within days. Ongoing surveillance of billions of doses over multiple years continues to confirm long-term safety. In contrast, COVID-19 infection can cause long-lasting complications (long COVID).

Concerns about unknown long-term effects are understandable when considering any new medical intervention. However, the nature of how vaccines work in the body, combined with extensive historical data from vaccine development and years of COVID-19 vaccine surveillance, provide strong reassurance about long-term safety.

Vaccines work by triggering a temporary immune response. The vaccine components—whether mRNA, viral vectors, proteins, or inactivated virus—are processed and cleared from the body within days to weeks. What remains is immune memory: the antibodies and memory cells that provide ongoing protection. This is fundamentally different from medications that remain in the body or accumulate over time.

Historical experience with vaccines shows that side effects typically occur within six weeks of vaccination. This is why regulatory agencies required at least two months of safety follow-up data before authorizing COVID-19 vaccines. In the years since initial authorization, ongoing surveillance has not revealed any new long-term safety signals.

In contrast, COVID-19 infection itself can cause long-lasting effects. "Long COVID" refers to symptoms that persist for weeks or months after the acute infection has resolved. These can include fatigue, cognitive difficulties ("brain fog"), shortness of breath, chest pain, and other symptoms. Studies suggest that vaccination reduces the risk of long COVID in breakthrough infections.