Prevention basics: Vaccinations

Vaccination offers the best protection against seasonal flu. The same is true for pandemic flu, if a vaccine is developed and available quickly enough. However, many people have reservations about the 2009 H1N1 pandemic flu vaccine, just as many people, for a variety of reasons, resist seasonal flu shots

The vaccine debate can be a heated one, and just like individuals, journalists are often at a loss when they quickly have to determine the motivations of those who speak on the issue. Public health officials overwhelmingly are likely to want people to get vaccinated. Some consumers, many of them well-organized on the web, are fearful about the side effects of vaccines. Pharmaceutical companies are motivated to sell vaccines, while statisticians will argue that there are risks and benefits to everything in life, but that mathematically, the benefits of vaccines by far outweigh the risks.

This section provides you with some essential information on the science and mechanics behind influenza vaccine development and production. It explains how vaccines work, how they are manufactured, how effective they are in different groups and who should or should not receive a flu vaccination. This page also sorts through some major misconceptions about vaccines.

In Pandemic Preparedness, we investigate the challenges of producing pandemic vaccines ahead of a second wave of the pandemic, while maintaining the greatest safety precautions possible. We also take a look at the challenges government faces when trying to communicate the vaccine’s benefits and safety to the public.

For a discussion of how people think and feel about risk (such as the risk of experiencing side effects from the vaccine versus the risk of acquiring the disease and facing complications) see our chapter on Crisis Communication, especially Peter Sandman’s contribution, Covering Risk.

Finally, we offer some background information on the massive vaccination campaign triggered by the 1976 swine flu scare that is so often quoted in the vaccine debate.

On this page...
  Vaccine basics »
  How do flu vaccines work? »
  How are flu vaccines made? »
  Seasonal influenza vaccines »
  How effective is the seasonal flu vaccine? »
  The 2009 H1N1 pandemic vaccine »
  Is the 2009 H1N1 vaccine safe? »
  Evaluating the risks »
  Common misconceptions about vaccines »

Vaccine basics

A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains a small amount of antigen, an agent that the immune system mistakes for (OR reacts to as though it were) an invading microorganism. This agent can consist of entire weakened or killed microorganisms, such as bacteria or viruses, or antigenic proteins those organisms produced.

Common types of vaccines are:

Attenuated: live viruses that have been weakened or modified so that they do not cause disease, but stimulate the body to make antibodies against the disease organism.
Inactivated: whole, but killed viruses that cannot reproduce or cause disease.
Subunit: fragments of virus (proteins or genetic material) that act as antigens to stimulate the body to make antibodies against the disease organism.

Vaccines often have added compounds known as adjuvants, chemical agents that stimulate immunity and can reduce the amount of the actual antigen needed in a dose. Some of the approved adjuvants include aluminum salts, lipid particles (liposomes), and oil-in-water mixtures (emulsions). The type used depends on the particular vaccine. Adjuvants are used in some vaccines approved for sale in the United States, but not in flu vaccines. (Several adjuvanted flu vaccines are sold in Europe.)

How do flu vaccines work?

When individuals are given vaccines, the antigens in the vaccine stimulate the body to produce antibodies. Antibodies are able to fight infection from viruses because they match the viral antigen(s): When a person is exposed to a virus, the antibodies attach to the matching antigen on the virus and prevent the virus from infecting cells in the body.

In other words, the agent in the vaccine stimulates the body's immune system to recognize the agent as foreign, destroy it, and “remember” it, so that the immune system can more easily recognize and destroy any of these same microorganisms it later encounters.

How are flu vaccines made?

Influenza vaccines are usually grown in fertilized chicken eggs. Manufacturers inject viruses (a “seed stock”) into millions of eggs and allow the virus to reproduce. Viruses destined for vaccine are isolated from the eggs, inactivated, and purified. It takes about six months to manufacture, safety-test and package the vaccine doses needed annually for seasonal flu, and it took about the same time to make similar amounts of 2009 H1N1 pandemic flu vaccine.

Current vaccine products have some limitations:

Vaccine production is not boundless—only a limited number of eggs are available.
Production is not always efficient—not all strains grow well in the eggs.
People with egg allergies cannot be vaccinated.

Production capacity is particularly important when seasonal flu and a flu pandemic coincide, as happened in fall 2009. Throughout the summer, manufacturers struggled to produce millions of doses of both seasonal and pandemic flu vaccines at the same time. Production ended up being sufficient to satisfy immediate orders from wealthier nations. (The 2009 H1N1 strain used for the vaccine turned out to grow well in eggs.) For poor countries, however, the situation is different, as explained in Global Challenges.

Newer manufacturing techniques that use mammalian cells rather than eggs to make vaccines are being developed to address growing concerns about the limitations of current production.

Seasonal influenza vaccines

Each year, seasonal influenza vaccines are produced from three strains of the influenza virus. These usually consist of two strains of influenza A and one strain of influenza B. Influenza C does not usually cause serious illness and is not included in the vaccines.

The most common types of influenza vaccines are injected. The alternative to a shot, the nasal spray vaccine contains live, weakened viruses that stimulate antibodies but do not cause illness. The nasal vaccine FluMist, sometimes called LAIV for “live attenuated influenza vaccine,” is approved for use in healthy people 2-49 years of age who are not pregnant.

Manufacturers produce a new vaccine each year because the predominant influenza virus that circulates in the population changes quickly. Mutations in the virus caused by a biological process called antigenic drift (see How Flu Viruses Change) make existing vaccines less effective or ineffective. Sometimes, one vaccine may have partial effectiveness against a related strain.

Influenza viruses circulate most prominently during the winter months. Investigators monitor the influenza strains dominant in the Southern Hemisphere’s winter to identify the influenza strains that are likely to cause illness during the next influenza season in the Northern Hemisphere. The selected strains are then used to manufacture seasonal influenza vaccine.

Given the time it takes to produce a vaccine, however, the strains for the vaccine have to be selected no later than the start of the influenza season in the Southern Hemisphere. Scientists make their best guess, and this carries the risk that the vaccine may no longer match the strains that will actually dominate the coming flu season. If the strains are mismatched, then the vaccine will be less effective, sometimes dramatically so.

Who can get the shot?

The seasonal influenza vaccine is approved for individuals older than 6 months, including healthy people and people with chronic medical conditions.

People who should not get the vaccine are:

Those allergic to eggs, since both the injectable and nasal flu vaccines are grown in them, and contain residual egg proteins.
Those with active infections or active diseases of the nervous system.

In case of shortages or increased demand, priority is given to children between 6 months and 19 years, individuals older than 50 years, and pregnant women.

How effective is the seasonal flu vaccine?

Seasonal influenza vaccines are 70 to 90 percent effective in preventing illness in healthy young adults. 1

For the elderly, a particularly vulnerable group, research shows:

The effectiveness of inactivated vaccines drops to between 30 and 70 percent for preventing infections and complications in people older than 65.
However, vaccinations reduce hospitalization by about 70 percent and death by about 85 percent among the elderly not in nursing homes.
Among those living in nursing homes, vaccines reduce the risk of hospitalization by about 50 percent, the risk of pneumonia by about 60 percent, and the risk of death by 75 percent to 80 percent.

Since only a few strains of influenza virus are used for making vaccines (see seasonal influenza above), people can still contract the flu after being vaccinated if they are infected with an influenza strain not included in the vaccine. Also, people sometimes get infected right after they are vaccinated but before they have developed enough antibodies. Full protection starts about two weeks after vaccination.

The 2009 H1N1 pandemic vaccine

The 2009 H1N1 vaccine distributed in the United States is made by several manufacturers from a single influenza strain dubbed influenza A/California/7/2009 (H1N1). Both injectable and nasal spray H1N1 vaccines are available. The vaccines do not contain adjuvants. The injectable vaccines are made in forms containing thimerosal, a mercury based preservative, or in pediatric formulations without the preservative.2 Although thimerosal has been debated as an agent capable of causing autism or brain damage, multiple scientific studies have shown no evidence of such a link.

The following four manufacturers’ vaccines have been approved by the FDA:

2009 Monovalent H1N1 Vaccine by CSL Limited. This is an inactivated vaccine manufactured on the same platform as their seasonal influenza vaccine, Afluria. It is injectable.
Monovalent H1N1 FluMist by MedImmune LLC. This is a live attenuated vaccine manufactured on the same platform as their seasonal vaccine, FluMist. It comes in the form of a nasal gel that is commonly called a spray.
2009 Monovalent H1N1 Vaccine by Novartis Vaccines and Diagnostics Ltd. This is an inactivated vaccine manufactured on the same platform as their seasonal vaccine, Fluvirin. It is injectable.
2009 Monovalent H1N1 Vaccine by Sanofi Pasteur Inc. This is an inactivated vaccine manufactured on the same platform as their seasonal vaccine, Fluzone. It is injectable.

The blog justthevax has a good summary of the details related to the 2009 H1N1 vaccines.

Injected vaccines can be used for those age 18 years old or older. The vaccine is given as a single shot. Since the pandemic vaccine supply is limited, priority is given to:

Pregnant women
Parents, siblings, and daycare providers of infants age 6 months or younger
Healthcare and emergency medical personnel
People between the ages of 6 months and 24 years
People between 25 and 64 years with medical conditions that place them at higher risk for influenza-related complications

Nasal sprays can be used in children age 2 to 9 years and children, adolescents, and adults age 10 to 49 years. One dose is sufficient for most patients, but children age 2 to 9 years require 2 doses.

Nasal spray vaccines should not be used in:

Children younger than age 24 months
Pregnant women
Nursing mothers
People older than 50 (it has not been tested in this age group)

The CDC provides frequent updates on vaccine supplies and priority recommendations.

Is the vaccine safe?

The 2009 H1N1 pandemic flu vaccines are expected to have a safety record similar to that of seasonal flu vaccines. The H1N1 vaccine manufacturers use the same processes as those for seasonal influenza vaccines. These methods have a long safety record as millions of doses have been used for many years. Also, the FDA and CDC constantly monitor adverse reactions to these vaccines and analyze available data frequently to quickly detect any serious side effects.

The Washington Post has some excellent reporting on the clinical trials and safety concerns.

The justthevax blog provides a good write-up of all four approved H1N1 vaccines as well as a fifth one that has not been approved yet, including clinical trial results and safety evaluations.

Evaluating the risks

One often-overlooked truth about influenza is that not all people are affected equally. Seasonal flu affects infants and the elderly more seriously than other groups, and the 2009 H1N1 pandemic affects young people more seriously than seasonal influenza does.

Since the first wave of the 2009 H1N1 pandemic hit, doctors, officials and scientists have been busy narrowing down which specific groups are most at risk to suffer serious complications from this particular strain. As is often the case with new infectious disease strains, there is a lot of guessing involved, and data sets are naturally small. But constant surveillance allows for important early observations.

As of October 2009, the novel H1N1 influenza virus rarely seems to be causing serious health outcomes. When that happens, however, it occurs most often among:

Healthy young people from birth through age 24
Pregnant women
Adults 25 to 64 who have underlying medical conditions

These observations inform decision making at the CDC and FDA, but they should also inform individuals and their thinking about the risks and benefits of vaccination. Every person has a unique set of circumstances. It is important to know if one’s risk for serious complications from the disease is higher or lower than the risk of serious side effects from the vaccine.

In “What You Need to Know About Swine Flu Vaccine” NPR’s Richard Knox provides a good example of how journalists can help people think through their personal risks.

Also, Ada Calhoun has an interesting story about how the vaccine discussion is confusing parents on the Web site doubleX.

As for official decision making, the CDC expects that the 2009 H1N1 pandemic vaccine will work in a fashion similar to the seasonal influenza vaccines, and that the benefits of vaccination will far outweigh the risks

Also, the Immunization Action Coalition, which is funded by the CDC, private foundations, and vaccine makers provides frequent updates on vaccines in the news.

Common misconceptions about vaccines

“Influenza is not a serious illness.” Influenza is a serious illness for many individuals. According to the CDC, at least 30,000 people, mostly infants and the elderly, die from it every year. Also at risk for serious complications are people who have other medical conditions such as asthma or diabetes. The new 2009 H1N1 virus causes serious illness in younger individuals. People born before the 1957 influenza pandemic may have some immunity to H1N1.3
“I remember vaccines from years ago. They had bad side effects.” Vaccines prepared from the 1940s to the 1960s in the United States contained some impurities that caused unpleasant side effects that older people may remember. Those side effects—headache, fatigue and fever—were so similar to the flu itself that many erroneously concluded the vaccine had given them the illness. However, newer vaccines are considered extremely safe and side effects have been reduced in most cases to soreness, redness, tenderness or swelling in the area where the shot was given.
“The vaccine causes influenza.” People cannot get influenza from the vaccine. If individuals come down with the flu after getting vaccinated, it is usually for one of the following two reasons:
- They were infected with a strain that was not included in the vaccine
- They were infected within two weeks of being vaccinated and their immune system had not made enough antibodies to the vaccine strains to fight off the actual disease.
  “Vaccines weaken the immune system.” Vaccination does not weaken immunity, it builds immunity. The seasonal vaccine protects against circulating strains of influenza, and the 2009 H1N1 vaccine protects against a new virus for which most people have no immunity.
  “Vaccines are not safe.” Vaccines are tested before they are used widely. Over the years, hundreds of millions of Americans have received seasonal flu vaccines. The most common side effects following flu vaccinations are mild, such as soreness, redness, tenderness or swelling in the area where the shot was given. As for the H1N1 pandemic vaccine, the FDA and CDC are constantly monitoring data as it comes in to detect any serious adverse reactions. In the trials and first weeks of H1N1 vaccinations, its safety record was as good as that of seasonal influenza vaccine.


  1. Jeffery K. Taubenberger, David M. Morens, “The pathology of influenza virus infections,” Annual Review of Pathology: Mechanisms of Disease 3 (February 2008): 499-522

  2. Federal Drug Administration, FDA approves vaccines for 2009 H1N1 influenza virus, Sept. 15, 2009.

  3. E. Pérez-Trallero, L. Piñeiro1, D. Vicente, et al.. “Residual immunity in older people against the influenza A(H1N1)—recent experience in northern Spain,” Eurosurveillance 14:39 (October 2009): 1-3.

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