Is the H1N1 Swine Flu Vaccine Safe?

H1N1 Swine Flu Vaccine Safety: Hype, Myths, and Facts

Medically Reviewed by Louise Chang, MD on October 14, 2009

What do we really know about the 2009 H1N1 swine flu vaccine? What do we really not know?

Questions about the safety of the vaccine persist. Surf the Internet or flip through TV stations and you'll encounter a multitude of myths and a whole lot of hype.

What are the facts? Straightforward answers follow these questions:

  • Is the 2009 H1N1 swine flu vaccine safe?
  • Isn't the 2009 H1N1 swine flu vaccine too new to trust?
  • Why should I believe what government scientists say about swine flu?
  • Does the H1N1 swine flu vaccine contain thimerosal?
  • The 1976 swine flu vaccine wasn't safe. Why should I trust this one?
  • Do we really know what drugmakers are putting in the swine flu vaccine?

No vaccine is 100% safe for everyone. People with allergies to eggs, for example, can't take flu vaccines because eggs are involved in the manufacturing process.

And flu vaccines cause mild but common reactions. About one in three people get a sore arm from the shot, some with a little redness or even swelling. Some 10% to 15% of people feel tired or get a headache; some may even run a low fever.

And vaccines can trigger rare but serious reactions, even among people with no apparent allergies or sensitivities.

So if vaccines aren't 100% safe, why risk them?

Approved vaccines -- including the 2009 H1N1 swine flu vaccine -- are calculated to be much, much less risky than the diseases they prevent. For example, out of every million people who get a flu shot, one or two will get a serious neurological reaction called Guillain-Barre syndrome (GBS).

But flu itself causes serious problems, including GBS, in far more than two in a million cases. And since a large proportion of the population will get swine flu, the vaccine risk is far smaller than the disease risk.

In clinical trials, 10,000 to 15,000 children and adults have received various manufacturers' brands of H1N1 swine flu vaccine. Nothing serious happened to any of them, including this reporter, who received a double dose of the Sanofi-Pasteur swine flu vaccine.

That's still not proof that no harm will come from the vaccine. Clinical trials cannot detect something bad that happens to one or two out of every 100,000 people vaccinated.

"There could be unknown side effects. Something could happen. But we think that is highly unlikely," says infectious disease and vaccine expert Mark Mulligan, MD, executive director of the Hope Clinic of the Emory Vaccine Center in Atlanta.

"The CDC, FDA, HHS [Health and Human Services Department], the Department of Defense, and several large HMOs with great medical records are all collaborating in enhanced surveillance for this national 2009 H1N1 vaccine campaign," Mulligan tells WebMD. "If there is a signal for a rare or late adverse event, we will identify it as early and as quickly as we can."

Is the swine flu vaccine brand new? Yes and no. The 2009 H1N1 swine flu vaccine is made exactly the same way as the seasonal flu vaccine, by the same manufacturers using the same materials -- except for one shiny new piece.

What has changed is the piece of the virus the vaccine uses to prime the immune system.

Vaccine experts tell WebMD this change isn't all that new. Every couple of years or so, a new variant of a seasonal flu virus comes along. When that happens, a "new" vaccine is made using the relevant part of the variant virus.

And even though the 2009 H1N1 swine flu is a genuinely new virus, it's still closely related to seasonal flu bugs. One of the vaccines in the three-in-one seasonal flu vaccine protects against seasonal H1N1 flu, which is about 75% similar to the 2009 H1N1 swine flu -- although it offers no protection against the pandemic flu.

Last year, some 100 million people got the seasonal flu vaccine. No safety issues appeared. That's reassuring, but it's no proof that something rare and unexpected can't happen.

There's no denying that the virus particle used in the vaccine has never been used before. No scientific calculation can rule out the chance that something unexpected might happen.

But one can calculate that this chance will be small. And the chance that the vaccine will prevent serious illness and deaths is very, very large.

The public health agenda is to promote healthy practices -- such as eating wholesome foods and quitting smoking -- that not everyone likes. Why? Science suggests that these policies save lives and cut health care costs.

The public health agenda also promotes vaccination against disease -- even though the rare individual is harmed by a vaccine. Why? Science suggests such a policy saves lives and cuts health care costs, as long as disease risks outweigh vaccination risks. A good example is the smallpox vaccine, which eradicated one of the scourges of mankind, even though a number of people were harmed by the vaccine.

After weighing the benefits vs. the risks, the health agencies of the U.S. government have launched the most massive vaccination campaign in history to fight the 2009 H1N1 swine flu. The CDC is using simple, direct messages -- including advertising and press conferences -- to encourage people to get the vaccine.

"In public health, you have to have campaigns and try to talk people into things," Mulligan tells WebMD. "The question about the government is a very important one. But things have changed since the old days of the Tuskegee syphilis experiment. Now we have very strict regulation of government research. People can trust and believe that this is not politicians talking, but researchers presenting evidence-based recommendations."

The 2009 H1N1 swine flu vaccine comes in three basic types: the FluMist nasal spray, single-syringe shots, and multi-shot vials.

Only the multi-shot vials contain thimerosal, a preservative that prevents bacterial contamination of the vial. Before thimerosal was added to vaccines, there were occasional vaccine injuries due to contamination.

Extensive study shows that there are no more adverse events in children or adults who receive thimerosal-containing vaccines than in those who do not.

But thimerosal contains a form of mercury. It's ethyl mercury, which is likely not as toxic as some other forms. Even so, nobody argues that mercury is good for your body. People who want to avoid thimerosal-containing flu vaccines must get the FluMist vaccine or the single-syringe shots.

Most people should have this choice. But single-syringe vaccines may not be available for every person in every location during every week of the vaccination campaign.

The 1976 swine flu vaccine was linked to safety issues. Neal Halsey, MD, director of the Institute for Vaccine Safety at the Johns Hopkins Bloomberg School of Public Health, was at the CDC in those days.

"We did identify an increased risk of GBS [Guillain-Barre syndrome] in the six weeks following immunization," Halsey tells WebMD. "What is not known at this time is exactly why that vaccine was associated with that increased risk."

No flu vaccine since then has been linked to this risk. Halsey thinks there is the potential for the H1N1 flu vaccine to carry a risk of causing one case of GBS per million people vaccinated.

"The theoretical risk of that rare complication has to be balanced against the severity of the H1N1 flu," he says. "There already have been a lot of deaths, many in otherwise healthy, normal children. There is always a real risk from the flu, and a theoretical risk from the vaccines."

In 1976, the risk from flu was theoretical, too. Despite a scary and fatal outbreak of an H1N1 swine flu at a military base, the virus never spread.

The 1976 H1N1 swine flu was a very different virus from the 2009 H1N1 swine flu, which combines elements from flu viruses that evolved in birds, humans, and pigs. And unlike the 1976 virus, the 2009 bug is causing a very real pandemic.

Vaccine labels are not easy to read. But they are made public by the FDA and other sources. If you want to know exactly what's in each kind of 2009 H1N1 swine flu vaccine, read the label. You can find all the labels here:

Show Sources


WebMD: "H1N1 Swine Flu Guide."

Neal Halsey, MD, director, Institute for Vaccine Safety, Johns Hopkins Bloomberg School of Public Health.

Mark Mulligan, MD, executive director, Hope Clinic, Emory Vaccine Center, Atlanta.

Institute for Vaccine Safety web site.

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