Understanding vaccine side effects

Attenuated vaccines introduce a weakened version of the virus into the body

Some vaccines, like those for measles, rely on weakened versions of a virus. These weakened pathogens are introduced into the body, triggering an immune response and helping the immune system learn to protect itself against stronger versions of the virus. (This also explains why some vaccines make you feel under the weather.)

Breaking down the different types of vaccine technologies

There are several different types of vaccines. Each type is designed to teach your immune system how to fight off germs—and the serious diseases they cause. This short overview breaks down the differences between the types.

What is vaccination? The World Health Org explains

Read up on the basics of vaccinations with this 101 from the World Health Organization, the public health arm of the United Nations.

The American Veterinary Medical Association advocates for vaccinations for pets to extend their lives

The American Animal Hospital Association provides guidelines, while many states legally require certain vaccines, such as rabies. Vaccinating healthy pets—even indoor-only pets—prevents outbreaks and protects other vulnerable animals, and adverse effects, such as allergic reactions and injection-site irritation, are rare. (Some readers may experience a paywall.)

Certain vaccinations may be more effective when given around the middle of the day

A 2023 analysis of the health records of more than 1.5 million people in Israel suggests that COVID-19 vaccines may be more effective when received around the middle of the day. Separate early research also suggests there may be similar trends for influenza vaccines.

Shingles vaccine may slow biological aging in older adults

Research published in 2025 that analyzed data from thousands of older adults found that those who had received the shingles vaccine exhibited signs of slower biological aging, such as lower levels of inflammation and slower epigenetic aging, than those who had not been vaccinated.

Before vaccines, there was 'variolation': what was it?

Before the technology behind vaccinations was discovered, some keen observers of medicine around the globe noticed how previous infections could impact future ones. Learn about the techniques early physicians used before they had vaccines.

A history of vaccine rollouts

History tells us the public will tolerate risk of harm from a vaccine if their fear of the disease exceeds their fear of the vaccine.

Secondary complications from measles in children range across the whole body

The most serious effects of a measles infection, particularly in young children, come from its attack on the immune system. This leaves the whole body vulnerable to follow-on diseases, with some of the most serious including respiratory conditions, encephalitis, and more.

Two festival-goers were hospitalized after receiving peptide injections, raising questions

Investigative journalism shop ProPublica wrote about two people who became extremely ill following antiaging peptide injections. Authorities were examining whether the individuals were sickened by an infection, contamination related to the injections, or an issue with the medication itself, according to documents obtained by the journalists at ProPublica.

The science of food allergies

Why do certain substances trigger an immune response while others don't? Listen to researchers explain the science in this five-minute video. Food allergies affect around 8% of children and 2% of adults in the US, and—unlike sensitivity or intolerance—they involve the immune system mistaking certain harmless proteins in foods as dangerous invaders, triggering a rapid and sometimes severe defense response.

See the evolution of Ebola vaccine and treatment development.

The COVID-19 pandemic led to a decrease in immunizations in the US and worldwide

Public health funds and resources that were diverted to COVID-19 response reduced efforts to combat polio and vaccinations for polio and other diseases declined during the pandemic, putting more people at risk for polio.

An overview of modern clinical research on LSD

Researchers have determined that LSD, taken in a medical setting, decreases anxiety related to "life-threatening disease," reduces brain reactivity to fearful faces, increases the meaning and emotional response derived from music, and more. No complications of LSD administration have been observed in medical settings, though acute adverse effects, including exhaustion, nausea, and difficulty concentrating.

See the evolution of Ebola vaccine and treatment development

The recently approved vaccine and treatments against Ebola are the result of many years of investments and have reshaped how the world addresses Ebola outbreaks. Four drugs were tested in the midst of the 2014 outbreak. Two proved to be so effective the clinical trial ended early. The vaccine proved effective during the 2018-2020 outbreak in the DRC.

Immunosuppressants enable organ transplants, but come with severe side effects

This drug cocktail helps to lower your body's natural response to attack a transplanted organ. Such medications usually need to be taken for a lifetime after receiving an organ and can come with serious side effects including high blood pressure, diabetes, increased risk of some cancers, and high cholesterol, among others.

mRNA vaccines alter immune response, not DNA, to combat diseases like cancer

These vaccines—developed using techniques to create COVID vaccines—are now being tested for cancer treatment and have shown improved survival rates in patients with lung cancer. Scientists believe the research involved in developing these vaccines has opened the door to new and effective personalized cancer treatments in the future.

See the differences between the Wegovy pill and injection

Examine this chart that compares and contrasts the oral and injectable Wegovy formulations. The differences include how they are absorbed by the body (through the digestive tract versus subcutaneously), their dosages, and the required timing (the pill is taken daily and has specialized timing requirements, while the injected form is administered once a week). Importantly, the pill form is also approved to treat fewer conditions.

Learn about the more than 100 known autoimmune diseases

Explore this resource with summaries and related resources for the dozens of autoimmune diseases defined in the medical literature. Many of these disease write-ups also have interviews with a patient with the disorder—offering a personal take about challenges, advice, and lifestyle effects.

What are zoonotic diseases?

Interspecies interactions are a breeding ground for disease transmission. This basic background information on zoonotic disease from the Centers for Disease Control and Prevention (CDC) chronicles the different ways humans can get sick, identifies what groups are particularly vulnerable, and offers links to other resources.

The negative effects of cannabis use

After decades of criminalization, cannabis is a legalized drug in an increasing number of countries around the world. This allows for accurate, robust research on the drug's effects on humans . While weed isn't as harmful as alcohol, regular usage has its negative effects. As strains of the drug become stronger, so does its consequences. Watch this video for a lucid look at the effects of cannabis on its users.

IVF was long considered controversial and ethically dubious

In spite of its wide acceptance as a form of reproductive assistance today, and its role in helping millions of people have children, in vitro fertilization was once considered a controversial threat to society. One magazine went as far as to describe the procedure as "the biggest threat since the atom bomb."

Support groups can offer insights into current and potential treatments

Beyond connecting with patients with similar experiences, support groups can also become centers for advocacy and for sharing research advances and medical experiences, including potential side effects from novel therapies or clinical trials.

Unlike milder fears, phobias may get in the way of proceeding with your life

Clinicians may recommend exposure therapy—exposing oneself to the thing triggering fears in some doses—as one way to help overcome a fear.

Doctors differentiate between a phobia and fear by asking patients whether their fear interferes with their day-to-day activities.

Listen to how medical error affected an emergency physician's approach

Emergency physician Dr. Sarah Gray discusses her worst medical error, what she learned from it, and how medical error and the culture around it lead to physician burnout. In addition to lawsuits, medical errors can lead to guilt, shame, anxiety, and fears of embarrassment, so this podcast discusses how sharing such errors and teaching about them can improve care.

Fantasy sports can have adverse health effects

Experts warn that fantasy sports, including fantasy football, can encourage obsessive use because they offer hits of dopamine. Daily fantasy games, which function more like gambling, are especially problematic.

Hookworm infection, though risky, may help fight certain autoimmune conditions by tamping down the immune system's inflammatory response.

Listen to an in-depth conversation on the links between Parkinson's disease and herbicide exposure and the challenging pace of scientific investigation.

See images of medical cases of firework injuries shared on this 'Instagram for doctors.' (Warning: Graphic content)

Read a deep dive on the 'all-purpose healing' peptide BPC-157.

Visualize how CRISPR-Cas9—a type of immune system used by single-celled organisms to attack invasive viruses—targets and makes precise changes to a gene's DNA.

Human-to-human transmission of hantavirus remains extremely rare, so the 2026 hantavirus cruise ship outbreak doesn't mean we're facing a COVID-style pandemic risk.

Poison ivy's rash-causing compound is found in the plant's roots, stem, and leaves—so coming into contact with any part of the plant can lead to rashes for the 85% of people who are allergic to it.

The tragic Pulitzer Prize-winning image 'The Vulture and the Little Girl' (1993) generated controversy, raised concerns about ethics in photography, and eventually led to the photographer's death.

After 25 people used virtual reality headsets to practice flying with virtual wings—flapping their arms—their visual cortex responded to the wings almost as if they were real arms, treating them as body parts.

Telehealth demand for GLP-1s for weight loss is booming as consumers seek easier access.

Radioactive elements can introduce software errors because the ionizing radiation they produce can detach electrons from atoms, and computer code relies on the distribution of electrons.

Larger animals like giraffes, elephants, and blue whales have developed a unique defense against cancer, a phenomenon known as Peto's Paradox

Children are now three times more likely to say they want to be a YouTuber when they grow up than an astronaut, with some even attending 'YouTube camps' that teach kids how to create content for the platform.

Alzheimer's can lead to brain shrinkage, with an average of around 2% of volume reduction per year

The deadliest aspect of volcanic eruptions is often not lava, but pyroclastic flows—avalanches of ash, gas, and rock that can move more than half the speed of sound.

20,000 letters to the advice columnist 'Dear Abby' provide insight into prevailing sex, religion, and LGBTQ fears, anxieties, and concerns from the 1950s onward.

Studies suggest when you sleep is as important as how much you get, and syncing with your circadian rhythms may lead to less fatigue and improved long-term health outcomes

Venture capital is a form of funding provided to early-stage companies with high growth potential, typically in exchange for an equity (or ownership) stake

A 2019 Facebook event called on attendees to "Storm Area 51, They Can't Stop All of Us," resulting in countless memes, the creation of a music festival, and eventually the arrests of those who did try to approach the area.

Some Pokémon have been named after real and fictional organisms, while some real-world species have been named after Pokémon, such as Chilicola charizard and Binburrum zapdos.

Memecoins are a type of cryptocurrency—a form of digital currency with a wide variety of uses—created around popular 'memes,' or viral internet jokes.

The Russia-Ukraine conflict is the largest ground war in Europe since World War II in terms of casualties, forces mobilized, and displacement of civilians.

Exploring the Larry Bird punch that almost changed March Madness