Vaccines save millions of lives every year. Yet some people resist being immunized. Here’s how vaccines work and why humankind needs them.

Decoder why are vaccines so important 1

Burial of four Ebola victims in Butembo, Democratic Republic of the Congo, 17 May 2019 (EPA-EFE/Hugh Kinsella Cunningham)

Measles is spreading across Europe. Ebola is spreading in the Democratic Republic of Congo. In both cases, health officials say vaccines are the answer to protecting people.

Why are vaccines so important? It comes down to one answer: They save lives.

Thanks to vaccines, smallpox no longer exists. A horrific disease, smallpox caused victims to develop blisters all over their bodies. It killed 30% of its victims in the 20th century, even with modern medical care. Vaccination wiped out smallpox in 1979, and now people are safe from this terrifying virus.

Almost everyone alive today has had at least a few vaccines. They’re given to babies and young children to protect them from some of the most deadly diseases on the planet: polio, diphtheria, tetanus, pertussis and the germs that cause pneumonia, to name just a few. About 85% of children have received some sort of vaccine, the World Health Organization (WHO) estimates.

The vaccines that most children in the world get today include immunizations against measles, mumps and rubella; polio; diphtheria and tetanus; pertussis or whooping cough; rotavirus; and germs that cause pneumonia. Some vaccines protect people for life, while others must be given every year or every few years.

Before vaccines, millions of people died every year from infections. Hundreds of millions more were crippled. Antibiotics help save lives, too, but they are useless against viruses such as polio, measles or yellow fever. And it’s clear that preventing disease is far better than treating someone for that disease. The WHO estimates that vaccines save 2 million to 3 million lives every year.

Vaccines are one of the safest medical interventions.

Before widespread vaccination, measles killed hundreds of thousands and even millions of people a year. As recently as 2000, measles killed 550,000 people, mostly young children. In 2016, measles killed fewer than 90,000 people.

Tetanus causes an agonizing death. Its nickname, lockjaw, describes the muscle spasms that often force a victim’s jaws to shut so tightly they can’t even be given water. It kills more than half of the people it infects. Without tetanus vaccination, the United Nations children’s agency UNICEF estimates that 1.2 million newborn babies would die every year.

Polio both kills and cripples. The virus attacks the nerves in the body, sometimes paralyzing victims so completely that they cannot even breathe for themselves and must spend the rest of their lives in beds, hooked up to breathing equipment. Polio infected 350,000 children in 1988, but only 33 in 2018, thanks to widespread vaccination.

A vaccine primes the immune system to recognize and fight a germ before it makes a person sick. The human immune system protects against many different bacteria and viruses every single day, but the germs that cause disease can overwhelm the body if they get a chance to get a foothold.

Vaccines are made using a part of the virus or bacteria that the immune system can recognize and prepare against, much like a police “wanted” poster can help law enforcement recognize a known criminal and be on alert.

The earliest vaccines were made using a “live” virus that may have been closely related to the targeted disease — like the cowpox virus used to inoculate against smallpox — or a weakened version that caused a mild infection that rendered a person immune to later infection. Sometimes bacteria and viruses are completely killed to protect people without causing an infection.

Most vaccines are now made either using harmless pieces of the germ or genetic material engineered in the lab to look like the bacteria or virus. They are often combined into a single dose so that people can be protected against several diseases with one injection. WHO says vaccines are one of the safest medical interventions.

Today, side-effects are extremely rare.

So why are there still any infectious diseases?

Some germs are difficult to make a vaccine against, such as the human immunodeficiency virus that causes AIDS. Some change constantly, making for a moving target — think influenza or flu or the rhinoviruses and adenoviruses that cause colds. Some, such as malaria, are caused by parasites that are more complex organisms than bacteria or viruses.

War and civil unrest can disrupt vaccination efforts, and people living in hard-to-reach places can go unvaccinated.

And some people are afraid of vaccines. They worry that vaccines may cause severe or even deadly side-effects, or rumors spread that vaccines are actually poisons devised to wipe out populations.

Neither fear is true.

Vaccines are the most-tested medical interventions because they are given to healthy people, so the benefit must greatly outweigh the risk. While it’s true that early vaccines could be dangerous — the live smallpox vaccine could make some people very sick, for example — modern vaccines are tested, re-tested and purified so that side-effects are extremely rare.

And there is no truth to the rumors that Western powers deliberately designed vaccines to cause AIDS or infertility.

But vaccine resistance has allowed measles to return to large parts of Europe. WHO reported 41,000 measles cases in Europe last year. Brazil had 10,000 cases, and Venezuela had more than 6,300 in 2018. Close to 1,000 people have been infected this year in the United States, thanks to pockets of vaccine resistance.

Ebola is less easily transmitted than measles, but far more deadly.

Measles can cause a range of symptoms, including fever and a characteristic rash. It can also lead to pneumonia and encephalitis, which can kill. Children who survive severe bouts of measles can become blind or partly paralyzed.

Measles kills one to two people out of every 1,000 infected, so epidemics that mount into the tens of thousands can take a severe toll. It’s the most infectious virus known. It spreads through the air, and nine out of 10 unvaccinated people who are exposed to measles will get infected.

Ebola is far less easily transmitted than measles, but it’s far more deadly. It kills anywhere between 20% and 90% of victims.

An epidemic in the West African countries of Sierra Leone, Guinea and Liberia between 2014 and 2016 infected more than 28,000 people and killed more than 11,000 of them. Now there’s an outbreak in the Democratic Republic of Congo that has infected at least 2,000 people and killed 1,300 of them.

Since 2014, two Ebola vaccines have been developed, and WHO and other groups are working to vaccinate people in the DRC. More than 100,000 people have been vaccinated in the campaign.

But it’s hard going because armed militias are interfering with efforts to vaccinate and to treat victims. And rumors are spreading that the vaccines are a curse, that they’re meant to enrich corporate interests, or that they are part of a plot to kill voters.

In fact, the Ebola vaccine has proved to be more than 97% effective, meaning that 97% of people who get vaccinated are protected against catching Ebola.


  1. What are some diseases that are prevented by vaccines?
  2. Why are some people afraid of vaccines?
  3. How are vaccines made?

Maggie Fox has been reporting on health and science for more than 20 years and is currently a consulting editor to Medscape and WebMD and a consultant on health and science news. She has covered conflict, politics and other international events from London, Hong Kong and Beirut. She has covered the cloning of Dolly the sheep, Ebola epidemics, stem cell technology, vaccine controversies and other stories for Reuters, CNN, National Journal and NBC News. She lives in Washington, DC.

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