From viruses to vaccines: Understanding the science, history and controversy

Viruses: Structure, Replication, and Evolution 

Viruses are composed of small pieces of genetic information, either DNA or RNA, enclosed in a protective shell known as the capsid. Some viruses also possess an additional lipid envelope, while those without an envelope are referred to as naked viruses. These viruses require a host to reproduce, and they can survive outside a host for a limited period until the capsid breaks down over time (Cleveland Clinic, 2023). 

Viruses are the smallest of all microbes in the living world. They can infect a variety of hosts, including humans, plants, bacteria, fungi, and animals, and they inhabit nearly every type of environment, from the air we breathe to the deepest depths of the ocean (Harvard Museum of Science & Culture, 2023). Some studies suggest that viruses may date back over 4 billion years, originating in a precellular world as self-replicating entities that evolved into parasites of other cells (Harvard Museum of Science & Culture, 2023). 

Although viruses are not classified as living cells, they carry genetic material (DNA or RNA) that enables them to hijack a host cell’s machinery to reproduce. This replication process often leads to infection. Viruses enter host cells via one of three mechanisms: 1) Receptor binding, which allows the virus to attach to specific receptors on the host cell surface; 2) Direct fusion, where the virus directly merges with the host cell membrane; or 3) the use of bacteriophages, which inject their genetic material directly into bacterial cells (Cleveland Clinic, 2023). 

Once inside the host cell, the viral genetic material is released, and it takes over the cell’s machinery to replicate. The host cell begins producing viral proteins and new viral particles. Eventually, these new viral particles are released from the host cell, often destroying the cell in the process, and can go on to infect other cells (Harvard Museum of Science & Culture, 2023). 

Characteristics of Viruses 

Scientists often describe viruses based on their shapes. The most common shape of viruses that infect humans is icosahedral or polyhedral, which is a geometric shape with many sides, resembling a soccer ball (Cleveland Clinic, 2023). This shape is commonly seen in viruses like adenoviruses, which are depicted in Figure 1. 

Virus Shapes and Structure 

Viruses can have various shapes, which scientists use to classify them. The most common shapes include: 

• Helical viruses: These viruses have a cylindrical shape, with their genetic material (DNA or RNA) coiled inside, much like a spring. 

• Spherical viruses: These are either helical or polyhedral viruses that are surrounded by an envelope, giving them a ball-like shape. 

• Complex viruses: These combine different shapes, such as a polyhedral “head” attached to a helical “body,” and are typically bacteriophages, which inject viral genetic material into bacterial cells. 

Size and Molecular Structure of Viruses 

Viruses are incredibly small and can only be observed using a powerful transmission electron microscope. Most viruses range in size from 20 to 400 nanometers, which is about 100 to 1,000 times smaller than a typical human cell (Cleveland Clinic, 2023). 

The size and molecular weight of viruses vary depending on the viral genetic material. Viral DNA serves as an instruction manual for building the virus, while RNA translates these instructions into a format that the host cell can understand, allowing it to make proteins (Cleveland Clinic, 2023).

Viruses can also exhibit different structural characteristics, such as: 

• Linear or circular genetic material 

• Positive-sense or negative-sense RNA 

• Single-stranded or double-stranded genetic material 

• Spike proteins, like those seen in the coronavirus, which are used for attaching to host cells. 

Viruses and Diseases

Viruses are responsible for a wide range of diseases, affecting humans and animals alike. Common viral infections include the common cold, flu, COVID-19, respiratory syncytial virus (RSV), chickenpox, measles, HIV/AIDS, HPV/genital warts, genital herpes, polio, rabies, mpox, Zika, hepatitis, and many other conditions that impact people worldwide. 

The scientific study of viruses and the diseases they cause is known as virology. Virologists are the scientists who specialize in researching viruses and their effects on health. 

History of Vaccines 

People in different parts of the world have historically tried to prevent illness by intentionally exposing healthy individuals to diseases. One early example is variolation, a practice used in the 15th century to prevent smallpox (WHO, A Brief History of Vaccination). The term “vaccine” comes from the Latin word vacca, meaning cow, as Dr. Edward Jenner used cowpox virus to develop the first known human vaccine (WHO). In 1796, Jenner used a live attenuated virus to develop immunity against smallpox (WHO). 

Vaccines are categorized based on how they are developed and the technology used. As mentioned, the first vaccine—Jenner’s smallpox vaccine—was a live attenuated vaccine. These vaccines use a weakened form of the virus to mimic a natural infection and produce a strong, long-lasting immune response, often providing lifelong protection after just one or two doses (HHS). However, people with weakened immune systems, chronic health conditions, or organ transplants should consult with their doctors before receiving live attenuated vaccines. A limitation of these vaccines is that they require refrigeration, which can pose challenges in countries without proper storage facilities. Common examples include MMR (Measles, Mumps, and Rubella), Rotavirus, Smallpox, Chickenpox, and Yellow Fever vaccines (HHS). 

By the late 1800s, inactivated vaccines, which use killed viruses or bacteria, were developed. The Typhoid (1896), Cholera, and Plague (1897) vaccines are early examples of this type (PMC). Inactivated vaccines provide some immunity, but are generally not as strong as live vaccines, often requiring booster doses. Common inactivated vaccines include Hepatitis A, Influenza, Polio, and Rabies (HHS). 

In the 1920s, scientists began developing toxoid vaccines using purified toxins, such as the Diphtheria toxoid (1923) and Tetanus toxoid (1926) (PMC). These vaccines target the toxins produced by pathogens rather than the pathogens themselves. Another group includes subunit, recombinant, polysaccharide, and conjugate vaccines, which use specific pieces of the virus or bacteria—like proteins or sugars—to trigger a strong immune response. These vaccines are safe for people with chronic health conditions, though they often require booster shots. Common examples include vaccines for Hib, Hepatitis B, HPV, Whooping Cough, Pneumococcal disease, Meningococcal disease, and Shingles (HHS). 

The most recent advancements include mRNA vaccines, such as those developed for COVID-19, and viral vector vaccines, which have been used for diseases like Ebola, Zika, Influenza, HIV, and COVID-19 (HHS). Scientists have worked on these technologies for decades. mRNA vaccines work by instructing cells to produce proteins that activate an immune response, while viral vector vaccines use a modified virus—such as adenovirus (which causes the common cold)—to deliver genetic material. These modern vaccines can be manufactured quickly and do not contain live viruses capable of causing the disease. 

Vaccine Controversy 

Vaccine controversy has existed since the first smallpox vaccine was introduced in the mid-to-late 1800s in England. Concerns included fears of scarring the flesh on a child’s arm and inserting lymph from the blister of a previously vaccinated individual. Some clergy even considered the vaccine “unchristian” because it came from an animal (History of Vaccines). 

Anti-vaccination sentiment was also fueled by distrust in medicine, doubts about vaccine efficacy, and concerns about personal liberty due to government-mandated vaccination laws—such as the Vaccine Act of 1853, which required smallpox vaccinations for infants up to three months old, and the 1867 Act, which extended this requirement to children up to 14 years old (History of Vaccines). 

In the 19th century, the United States experienced a smallpox outbreak that led to vaccination campaigns and the rise of organized anti-vaccine activity. The first U.S. anti-vaccination society was founded in 1879 (History of Vaccines). 

The Diphtheria, Tetanus, and Pertussis (DTP) vaccine faced controversy in the mid-1970s over alleged links to neurological conditions. A series of case reports and the National Childhood Encephalopathy Study (NCES) in the UK examined this claim but ultimately found that the risk was extremely low. This led to a pro-immunization campaign (History of Vaccines). In the U.S., the controversy grew in 1982 due to books and documentaries describing supposed vaccine injuries and downplaying the benefits of immunization. 

In 1998, British doctor Andrew Wakefield published a study suggesting a link between the MMR (Measles, Mumps, and Rubella) vaccine, bowel disease, and autism. This study fueled widespread public fear despite a lack of scientific support (History of Vaccines). In 2004, the journal The Lancet, which originally published Wakefield’s work, retracted the study, citing conflicts of interest and ethical concerns. By 2011, Wakefield was found guilty of scientific fraud for falsifying data and profiting from his claims. Numerous studies have since confirmed the safety of the MMR vaccine, with no link to autism (History of Vaccines). 

A common topic of controversy is thimerosal, a mercury-containing compound used as a preservative in some vaccines. In 1999, health organizations and manufacturers agreed to reduce or eliminate thimerosal from vaccines as a precaution. Today, it is no longer used in most childhood vaccines, although it may still be present in some flu vaccines. Over 30 years of research have shown no clear link between thimerosal and autism, ADHD, or developmental delays (History of Vaccines). 

More recent conspiracy theories claim that mRNA vaccines can alter DNA, cause infertility, or contain microchips. These myths are scientifically inaccurate. mRNA vaccines provide temporary instructions for cells to produce antibodies and do not enter the cell nucleus where DNA is stored. The mRNA degrades naturally within days or weeks (History of Vaccines). Claims that vaccines contain tracking devices are also false—mRNA vaccines contain harmless ingredients like lipids, salts, and sugars, and the small amount of liquid (0.3–0.5 mL) makes it impossible to include tracking technology. 

Clinical trials have shown no difference in fertility rates between vaccinated and unvaccinated individuals. Pregnant individuals also face low risk from the vaccine, and the benefits during pregnancy outweigh any potential risks (History of Vaccines). 

Concerns about ingredients like formaldehyde and aluminum are also exaggerated. A pear contains about 13 times more formaldehyde than any vaccine, and breastfed infants consume 100 times more aluminum than is found in a vaccine dose (History of Vaccines). Aluminum salts in vaccines help enhance the immune response and prevent contamination. 

Finally, some argue that “herd immunity” makes personal vaccination unnecessary. However, decreasing vaccination rates—especially for the MMR vaccine—are causing the return of preventable diseases like measles. Herd immunity requires at least 95% vaccination coverage, which is not currently being met due to increasing vaccine hesitancy (History of Vaccines). 

Current Measles Outbreak 

 In 2000, the United States declared that measles had been eliminated. However, outbreaks have re-emerged, primarily due to unvaccinated international travelers bringing the virus into the country (CDC, 2024). As of April 17, 2025, the Centers for Disease Control and Prevention (CDC) reported 800 confirmed measles cases across 25 states. In contrast, only 285 cases were reported in all of 2024, though they occurred in 33 states (CDC, 2025). Globally, measles continues to be a serious health concern. In 2023, an estimated 10.3 million people were infected with measles across all world regions (CDC, 2024). 

 The MMR (measles, mumps, and rubella) vaccine is recommended for children at 12 to 15 months of age, with a second dose between ages 4 and 6 (CDC, 2024). The first dose of MMR is about 93% effective, while the second dose boosts effectiveness to 97% (CDC, 2024). 

 If you have already received the recommended two doses, you generally do not need to be revaccinated unless you are in a high-risk group. For older adults who received childhood MMR vaccines and are uncertain about their immunity, a simple blood test—MMR antibody screening (IgG)—can determine whether you have protective antibodies. It is best to consult with your healthcare provider before seeking additional MMR vaccination or requesting antibody testing. They can help evaluate your individual risk factors and guide you on whether vaccination is necessary, especially if you missed childhood immunizations. 

Conflict of Interest Statement:
I have no conflicts of interest regarding vaccines. I do not work for any vaccine company, nor do I personally own stocks in vaccine-related pharmaceuticals, beyond those typically held in a standard investment portfolio. This article was written with the goal of educating readers about vaccines and offering support in making an informed decision, without any personal bias or financial gain. 

References: 

1. Cleveland Clinic. (2023). Virus. Retrieved from https://my.clevelandclinic.org/health/body/24861-virus
2. Harvard Museum of Science & Culture. (2023). World of Viruses. Retrieved from https://hmsc.harvard.edu/online-exhibits/world-viruses/ 
3. U.S. Department of Health and Human Services. (n.d.). Types of vaccines. HHS.gov. Retrieved from https://www.hhs.gov/immunization/basics/types/index.html
4. World Health Organization. (n.d.). A brief history of vaccination. WHO. Retrieved from https://www.who.int/news-room/spotlight/history-of-vaccination/a-brief-history-of-vaccination
5. Plotkin, S. A., & Plotkin, S. L. (2011). The development of vaccines: How the past led to the future. Nature Reviews Microbiology, 9(12), 889–893. https://pmc.ncbi.nlm.nih.gov/articles/PMC4151719/ 
6. College of Physicians of Philadelphia. (n.d.). History of anti-vaccination movements. History of Vaccines. Retrieved from https://historyofvaccines.org/vaccines-101/misconceptions-about-vaccines/history-anti-vaccination-movements
7. College of Physicians of Philadelphia. (n.d.). Top ten anti-vaccine myths – debunked again. History of Vaccines. Retrieved from https://historyofvaccines.org/blog/top-ten-anti-vaccine-myths-debunked-again
8. Centers for Disease Control and Prevention. (2024). Measles data and statistics. Retrieved April 20, 2025, from https://www.cdc.gov/measles/data-research/index.html 
9. Centers for Disease Control and Prevention. (2024). Measles, mumps, and rubella (MMR) vaccine. Retrieved April 20, 2025, from https://www.cdc.gov/measles/vaccines/index.html?CDC_AA_refVal=https%3A%2F%2Fwww.cdc.gov%2Fvaccines%2Fvpd%2Fmmr%2Fpublic%2Findex.html