2009 H1N1 Influenza Vaccine Safety

by Anthony Fiore, MD, MPH, medical epidemiologist for the Influenza Division of the CDC.



Many people are just getting up to speed on the 2009 H1N1 monovalent vaccines, and it's quite natural to have questions about them, including about safety. It's confusing enough just to keep up with the recommendations for seasonal flu vaccine and all the other things that are recommended to promote good health. Let's consider a few of the questions I have been getting.

Is the 2009 H1N1 influenza vaccine new?

The answer is that the virus is new, but the vaccine against this virus is being made in exactly the same way, by the same manufacturers, in the same facilities, and undergoing the same pre-release testing as is done for seasonal influenza vaccines every year. The injected 2009 H1N1 vaccine (flu shot) is made just like the annual flu shot, and contains killed viruses that cannot cause flu. The nasal spray 2009 H1N1 vaccine is made exactly the same way as the seasonal nasal spray vaccine. The nasal spray vaccines contain influenza viruses that have been weakened so that they cannot cause influenza. These seasonal flu vaccines have been used for many years, and have an excellent safety record.

What are "adjuvants"? And are they used in the 2009 H1N1 flu vaccine?

Adjuvants are another thing people have been asking me about. Adjuvants are ingredients added to some vaccines (for example, DTP, a vaccine that protects against diphtheria, pertussis and tetanus) that increase your body's immune response, and they are vital to how many other critical vaccines work. But influenza vaccines available in the United States – seasonal and 2009 H1N1 – do not contain adjuvants.

So why isn't the 2009 H1N1 virus in the annual flu vaccine?

The answer to this is that it would have been, if the virus had arrived a few months earlier. Seasonal flu vaccines are made in a yearly cycle, and the process must start in February each year in order to make enough for the fall vaccination season. Because the 2009 h1N1 virus started making people sick after seasonal vaccine was already being made the U.S. government decided to make a separate vaccine to protect against 2009 H1N1.

Was the vaccine rushed out too quickly, without the usual influenza vaccine safety testing?

Absolutely not. The same steps to ensure safety that we use each year for seasonal influenza vaccines were used for the 2009 H1N1 vaccines. Vaccine experts expect the 2009 H1N1 influenza vaccine to have a similar safety profile as seasonal flu vaccines, which have a very good safety track record. Over the years, hundreds of millions of Americans have received seasonal flu vaccines.


So the 2009 H1N1 is made the same way as seasonal vaccines- but what side effects are being seen in the clinical trials?

The clinical trial results are already in or are being analyzed, and some have already been published or reported. You will be seeing more results published over the next few weeks. Just in the past few days, the National Institutes of Health has released some data from the trial conducted among pregnant women and children, and its all good and expected news. The vaccine is very good at stimulating a protective immune response. People participating in the clinical trials so far are having the usual minor and short term side effects that we see from seasonal influenza vaccines. This means that for the flu shot, some people will have arm soreness for a day or two; some will have a sense of fatigue or a headache the day they are vaccinated. Most didn't report any side effects. For the nasal vaccine, people have reported runny nose, headache, or sore throat – all mild symptoms that resolve after a day or so. Again, many didn't have any reaction to the nasal vaccine. If you put the types of side effects that people report after getting seasonal flu vaccine side by side with those that have been seen among person who have received a 2009 H1N1 vaccine, you would not be able to tell them apart.

Do the vaccines have preservatives?

Some do – those that are formulated to be in multidose vials. Thimerosal is a very effective preservative that contains a small amount of mercury. Influenza vaccines in multidose vials contain 12.5 to 25 micrograms of mercury per influenza vaccine dose. Thimerosal has been used since the 1930s to prevent contamination in some multi-dose vials of vaccines (preservatives are not required for vaccines in single dose vials or syringes). The Advisory Committee on Immunization Practices recently stated that "No scientific evidence indicates that thimerosal in vaccines, including influenza vaccines, is a cause of adverse events other than occasional local hypersensitivity reactions in vaccine recipients. In addition, no scientific evidence exists that thimerosal-containing vaccines are a cause of adverse events among children born to women who received vaccine during pregnancy". For those who remain concerned about thimerosal, preservative-free inactivated influenza vaccines are available, and the live attenuated vaccine does not contain a preservative either. The total amount of inactivated influenza vaccine available without preservatives will continue to increase in the coming weeks.

How is vaccine safety monitored after the clinical trials?

The clinical trials have included thousands of persons with no reports of severe side effects. Based on our long experience with influenza vaccines, and the information from the trials, we expect that any serious side effects following vaccination with the 2009 H1N1 influenza vaccine will be rare. But CDC and FDA, working with health departments, healthcare providers, and large healthcare systems will closely monitor the safety of this vaccine using both the usual safety monitoring systems that are already up and running, as well as additional systems that use data sources we have not used before. CDC is also working with professional organizations whose members might be more likely to see a particular kind of side effect to be sure providers know how to rapidly report anything that they are concerned about. This is being done to assure the public that safety is being taken seriously, because if people don't completely trust the way safety is being assessed, they might be hesitant to take advantage of the protection that vaccination can offer. Furthermore, we also need to respond to any hint that there is a problem - because when you vaccinate tens of millions of people its inevitable that some health event will happen to somebody shortly after vaccination. The safety monitoring systems will be critical to helping us figure out whether whatever happened was an unrelated chance event (like getting hit by lightening after getting a haircut), or possibly connected to vaccination.

Who do I tell if I think I might have had an unexpected vaccine side effect?

You or your doctor can report what happened through the Vaccine Adverse Event Monitoring System or VAERS, which takes reports from any person or clinician who has some health event happen to them after vaccination (www.vaers.hhs.gov). You don't have to prove that the event is related in order to report, because the purpose of this system is detect possible safety signals that need further investigation.

I am convinced that vaccine is safe for healthy people, but what about those who are more vulnerable, like young children, pregnant women, or persons with weak immune system illnesses?

Vulnerable persons such as pregnant women, young children, and persons with weakened immune systems have taken seasonal flu vaccines for years and have not had problems with vaccine safety. There are certain people who should get the flu shot instead of the nasal spray vaccine, such as pregnant woman and people with certain medical conditions. But young children, pregnant women, or people with weak immune systems are at particularly high risk for 2009 H1N1 influenza complications, and are among those who should be vaccinated first. Furthermore, it's also quite safe to get either form of the 2009 H1N1 vaccine if you live with or provide care for people who are more vulnerable. The one exception is if you have contact with or provide care for hospitalized patients in special protected environments like bone marrow transplant units. While most health care workers don't do this as part of their usual job, those who do should get the flu shot instead of the nasal spray vaccine.

Vaccine safety has always been taken very seriously at CDC, and the public and health care providers should feel assured that all evidence so far indicates that the 2009 H1N1 vaccines will have the same excellent safety record as seasonal vaccines. This vaccine is the key to protecting people against 2009 H1N1 flu. I hope the information I've provided in this blog helps those of you who are recommended to get it to make an informed decision. As a doctor, I recommend that anyone who is at high risk for influenza complications and anyone who wants to avoid getting 2009 H1N1 influenza and passing it on to others, be vaccinated as soon as they can. There should be much more vaccine available in the coming weeks