The opening tweet of tsharp’s thread, posted on 6 July 2018.
In July 2018 a student who tweeted as tsharp (@taaylor_shaarp) turned her senior paper on vaccination into a Twitter thread. In about twenty short posts she walked through the diseases vaccines prevent, how herd immunity works, why the autism scare does not hold up, and what is really in a vaccine. She ended with her full list of works cited. The opening tweet alone collected more than 47,000 likes.
The thread worked because it was plain and patient. Each tweet tackled one claim, used simple numbers and pointed back to sources such as the US Centers for Disease Control and Prevention (CDC). Many replies came from parents, including people caring for children with weak immune systems, who thanked her for putting the facts in one place.
Below you will find every tweet in the thread, each with a short note on the science behind it. Where a figure has changed since 2018, or where the wording oversimplifies, we say so and link to a reliable source. Nothing here replaces medical advice: if you have questions about a specific vaccine or your own child’s health, talk to a doctor or nurse.
The diseases vaccines prevent
The author of the thread in a profile photo. She explained that the thread was based on the senior research paper she had written on vaccination, which is why it came with a full bibliography.
The thread’s opening line as it appeared on Twitter, with tens of thousands of likes and replies. The informal tone (“let me hit you with the facts real quick”) is part of why it reached people who would never open a medical leaflet.
She lists twelve childhood diseases covered by routine vaccines in the United States, from chickenpox to Hib. The exact list depends on the country’s schedule, but all of these infections once caused large numbers of hospital stays and deaths among children.
A key point: diseases that are rare in one country are still common elsewhere and can arrive with a single traveller. That is how most modern measles outbreaks in the US and Europe begin, with unvaccinated people as the ones most likely to catch it.
The whooping cough figure is accurate. The CDC recorded 48,277 cases of pertussis in 2012, the highest annual number since 1955. Waning immunity from the newer acellular vaccine and gaps in coverage both played a part, and babies too young to be fully vaccinated face the greatest risk.
Measles is one of the most contagious diseases known. The CDC estimates that up to nine in ten non-immune people who are close to an infected person will catch it, which is why very high vaccination coverage is needed to stop it spreading.
Strictly speaking, measles was declared eliminated from the United States in 2000, meaning it no longer spread continuously within the country, not that it had vanished. Since then outbreaks linked to clusters of unvaccinated people have returned, with large ones in 2019 and again in 2025.
How herd immunity protects the vulnerable
A simple definition of herd immunity, also called community immunity. When enough people are immune, an infection cannot find enough new hosts to keep spreading, so even those who are not immune are less likely to meet it.
The “hot spots” point is well documented. National vaccination rates can look healthy while individual schools or neighbourhoods fall far below the level needed, and those pockets are where outbreaks take off.
Some people cannot be vaccinated or do not respond well to vaccines: newborns, people having chemotherapy, organ transplant recipients and others with weakened immune systems. They depend on the people around them being protected.
Her example of a child with leukaemia reflects a real clinical problem. Children in cancer treatment often cannot receive live vaccines such as MMR, and measles can be life-threatening for them.
The herd-of-animals image is a helpful way to picture the idea: a protective ring around those who cannot defend themselves. Public health experts sometimes use the related term “cocooning” for vaccinating everyone around a newborn.
Do vaccines cause autism? What the evidence says
The most-liked tweet in the thread, and the bluntest. Its main point is right: large studies have found no link between vaccines and autism. The wording needs one correction, though. Autism is a lifelong developmental difference, not a “mental disability”, and research shows it has strong genetic roots and is shaped before birth.
Here she explains odds ratios, the measure used in many of the studies she read. An odds ratio of 1 means no difference between groups; below 1 means the outcome was slightly less common among the vaccinated.
The figure of 0.84 for MMR comes from a 2014 meta-analysis in the journal Vaccine that pooled studies covering more than a million children and found no link with autism. A lower odds ratio there is best read as “no increased risk”, not as proof that vaccines prevent autism.
The autism scare began with a 1998 paper in The Lancet by Andrew Wakefield and colleagues, based on just 12 children. Journalist Brian Deer later showed that data had been altered, The Lancet fully retracted the paper in February 2010, and Wakefield was struck off the UK medical register that May. Since then, studies of hundreds of thousands of children in Denmark and elsewhere have found no difference in autism rates between vaccinated and unvaccinated children. Signs of autism tend to become noticeable at around the age when several vaccines are given, which is why the two are easy to confuse.
What is actually in a vaccine
She turns to ingredients, the source of many fears. Vaccines contain the active component (a weakened or inactivated germ, or part of one) plus small amounts of substances that keep them stable, safe and effective.
The mercury point is correct. Thimerosal contains ethylmercury, which the body clears far faster than methylmercury, the form that builds up in some fish. The two should not be treated as the same chemical.
This needs some updating and one correction. In the US, thimerosal was removed from routine childhood vaccines by 2001 as a precaution, and only some multi-dose flu vials kept it; in 2025 the US moved to stop using thimerosal in flu vaccines altogether. Thimerosal is a preservative that stops contamination, while inactivating viruses is the job of other substances such as formaldehyde.
Formaldehyde is made naturally by our own metabolism and is always present in the blood. The exact multiple quoted varies, but the Children’s Hospital of Philadelphia estimates that a young baby’s blood already holds between 10 and 220 times more than any single vaccine contains.
Aluminium salts are used as adjuvants to help the immune system respond. Babies also take in aluminium from food: according to the Children’s Hospital of Philadelphia, the amount over the first six months ranges from a few milligrams for breastfed babies to over 100 for soy formula, far more than the total from vaccines.
Her conclusion is that skipping vaccines carries greater risk than getting them. Vaccines can have side effects, mostly mild ones like a sore arm or a fever, and serious reactions are rare; health agencies monitor them continuously.
The first page of her works-cited list, which includes the CDC, the US Department of Health and Human Services, the American Academy of Pediatrics and peer-reviewed studies. Sharing sources openly is what set this thread apart from most viral health posts.
A follow-up three days later, posted in reply to people still arguing. She explains that she used news sites such as Time only for her introduction and conclusion, and relied on medical sources for the facts.
How people responded to the thread
The replies were overwhelmingly positive. Several came from people with a personal stake in high vaccination rates.
A supporter hoping more people would read the whole thread. Long threads often lose readers after the first few tweets, so encouragement like this helped keep it circulating.
Another reader praising how clearly the thread was written. Clear writing matters in health communication, because confusing explanations leave room for rumours to fill the gaps.
One of the most telling replies: a young woman from a family opposed to vaccination who said the thread helped her research the question. Studies on vaccine hesitancy suggest that calm information from a peer can be more persuasive than official warnings.
A reader predicting, with a laughing emoji, that critics would call the facts fake. Online health debates often split along exactly these lines.
A brief show of respect for the author. Thousands of replies like this one pushed the thread into many people’s timelines.
An enthusiastic reply with a mild swear word and syringe and pill emojis. Humour and informality were part of the thread’s appeal for younger audiences.
A reader thanking her for being funny and kind as well as informative. Tone matters: research on persuasion suggests people are more open to facts when they do not feel attacked.
This reply makes an important scientific point: early signs of autism often appear around the same age that children receive several vaccines. Two things happening at the same time does not mean one caused the other, and that is exactly what large studies were designed to test.
The mother of a child with a weakened immune system thanking her. Families like hers rely on herd immunity because their children may not be able to get some vaccines themselves.
A new mother who had already vaccinated her baby, saying the thread finally gave her the reasons behind her choice. Many parents follow the schedule without ever hearing the evidence explained.
A reply from the wife of a transplant patient, who takes medicines that suppress the immune system. For people like him, an outbreak in the community can be dangerous.
A final cheer for the author’s research. The thread is still a good example of how a young person used her own school project to explain science to a huge audience.
Why vaccine myths keep coming back
Vaccines are, in a sense, victims of their own success. Most parents today have never seen a child struggle to breathe with whooping cough or be left disabled by measles, so the risks of the diseases feel distant while worries about the injection feel immediate. Social media then rewards dramatic personal stories over careful statistics.
Threads like this one show that the answer is not to mock people with questions. Explaining one point at a time, citing sources and admitting where wording needs correcting builds trust. If you are unsure about a vaccine for yourself or your child, the best next step is a conversation with a doctor, nurse or pharmacist who can talk through your specific situation.
Sources: Lancet MMR autism fraud, Wikipedia; Vaccines and autism, Wikipedia; Thiomersal and vaccines, Wikipedia; Children’s Hospital of Philadelphia: Aluminum in vaccines; Children’s Hospital of Philadelphia: Formaldehyde in vaccines; BMJ: Lancet retracts Wakefield’s MMR paper (2010)
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