Global Alert and Response (GAR)

Safety of pandemic (H1N1) 2009 vaccines

30 October 2009

Safety

Are pandemic vaccines safe?

Outcomes of studies completed to date suggest that pandemic vaccines are as safe as seasonal influenza vaccines. Side effects seen so far are similar to those observed with seasonal influenza vaccines.

What about safety for pregnant women?

To date, studies do not show harmful effects from the pandemic influenza vaccine with respect to pregnancy, fertility, or a developing embryo or fetus, birthing or post-natal development. In view of the elevated risk for severe illness for pregnant women infected by the new influenza, in clinical studies, pregnant women are a group that should be vaccinated against infection, as supplies allow.

Recent studies show that infected pregnant women have a 10 times higher chance to require hospitalization in intensive care units than infected persons in the general population, and 7% to 10% of hospitalized cases are women in their second or third trimester of pregnancy. The benefits of vaccination far outweigh the risks.

Additional studies on pregnant women following immunization are continuing.

What about my child's safety from a reaction?

The most frequent vaccine reactions in children following influenza immunization are similar to those seen after other childhood immunizations (such as soreness at the injection site, or fever). A child's health care provider or vaccinator can advise on the most appropriate methods for relief of the symptoms. If there are concerns about a child's safety from a reaction, consult a health care provider as soon as possible. Please note that a child may suffer from a condition not related to immunization, which coincidentally developed after vaccination.

Testing and approval

What kind of testing is being done to ensure safety?

Because the pandemic virus is new, both non-clinical and clinical testing is being done to gain essential information on immune response and safety. The results of studies reported to date suggest the vaccines are as safe as seasonal influenza vaccines. However, even very large clinical studies will not be able to identify possible rare events that can become evident when pandemic vaccines are administered to many millions of people.

WHO advises all countries administering pandemic vaccines to conduct intensive monitoring for safety and report serious adverse events.

Who approves pandemic vaccines for use?

National authorities for medicines approve (or license) pandemic influenza vaccines for use. These authorities carefully examine the known and suspected risks and benefits of any vaccine prior to its licensing. Expedited regulatory processes in some countries have helped to license the new vaccine in a timely manner. However, the testing and manufacturing processes for the new vaccines are similar to seasonal influenza vaccines to ensure quality and safety.

Side effects

What are the expected side effects of the new vaccines?

Some side effects can be associated with influenza vaccination. How often they result depends on the type of vaccine, how it is administered, and the age of the vaccine recipient. There are two main types of vaccines: one is manufactured with inactivated viruses, the other uses live viruses.

Inactivated vaccines, administered by injection, commonly cause local reactions such as soreness, swelling and redness at the injection site, and less often can cause fever, muscle- or joint- aches or headache. These symptoms are generally mild, do not need medical attention, and last 1 to 2 days. Fever, aches and headaches can occur more frequently in children compared to elderly people.

Rarely, such influenza vaccines can cause allergic reactions such as hives, rapid swelling of deeper skin layers and tissues, asthma or a severe multisystem allergic reaction due to hypersensitivity to certain vaccine components.

Live vaccines are given via a nasal spray, and can commonly cause runny nose, nasal congestion, cough, and can less frequently cause sore throat, low grade fever, irritability and head- and muscle- aches. Wheezing and vomiting episodes have been described in children receiving live influenza vaccines.

Have clinical studies identified all the possible side effects?

Again, even very large clinical studies will not be able to identify possible rare events that can become evident when pandemic vaccines are administered to many millions of people. These can only be assessed when a vaccine is in widespread use.

Clinical trials often provide safety information for the general population. Additional monitoring of some special groups of vaccine recipients is necessary to gather specific safety information.

Additional and comprehensive monitoring efforts of the pandemic influenza vaccine are being planned as they are being used by more and more people around the world.

WHO advises all countries administering pandemic vaccines to conduct intensive monitoring for safety, and report adverse events.

Adverse events

Have their been any reports of serious reactions, or adverse events, to pandemic vaccines?

As of late October, there is no indication at this stage that unusual adverse events are being observed after immunization, according to clinical trials and adverse event monitoring during deployment of vaccines in early introducer countries. The need for continued vigilance and regular evaluation by health authorities is ongoing.

How should serious reactions to the vaccines be reported?

Reports of serious adverse events, and those raising concerns, should always be submitted to national authorities. So far, reports of potential adverse events following immunizations have been well notified to authorities.

What happens when an adverse event is reported?

At the national level, individual reports are scrutinized for completeness and possible errors. In some instances, reports need to be validated and additional details must be checked. Reports are analysed for findings that are expected or appear more frequently than expected. If an analysis indicates a potential problem, further studies and evaluation are conducted and all relevant national and international authorities are informed. Decisions for appropriate measures are then made to ensure continuing safe use of the vaccine.

Risks falsely associated with vaccines

Will pandemic vaccines contain thiomersal, which some believe is a risk to health?

Thiomersal is a commonly used vaccine preservative to prevent vaccine contamination by bacteria during use. Inactivated vaccines will contain thiomersal if they are supplied in multi-dose vials. Some products can have ''traces" of thiomersal when the chemical is used during the production process as an antibacterial agent, which is later removed during the purification process.

Thiomersal does not contain methyl mercury, which is a naturally-occurring compound and whose toxic effects on humans have been well studied. Thiomersal contains a different form of mercury (i.e. ethyl mercury, which does not accumulate, is metabolized and removed from the body much faster than methyl mercury).

The safety of thiomersal has been rigorously reviewed by scientific groups. There is no evidence of toxicity in infants, children or adults, including pregnant women, exposed to thiomersal in vaccines.

Why do some pandemic influenza vaccines contain adjuvants and others don't? Are vaccines with adjuvants a health risk?

Adjuvants are substances that enhance the immune response in vaccines and can make them more effective. They have been used for many years in some vaccines. Scientific data support the safety of adjuvants in pandemic influenza vaccine production.

Some seasonal influenza vaccines that are intended for people known to have poor immune responses to immunization contain an adjuvant. Some pandemic vaccines contain an adjuvant to reduce the amount of virus antigen to be used (an antigen is a substance capable of stimulating an immune response).

Manufacturers decide whether a product is formulated with or without an adjuvant. Adjuvants used with pandemic influenza vaccines are already licensed for use with other vaccines (e.g. hepatitis B, seasonal or pandemic influenza vaccines, or others), and have a safe track record.

Can influenza vaccination cause chronic diseases?

Current evidence does not indicate that seasonal influenza or pandemic influenza vaccines, or any other vaccine against novel human influenza viruses, either induce or aggravate the course of chronic diseases in vaccine recipients. Careful assessment is required to clarify if adverse events that occur after vaccination are actually caused by an influenza vaccination.

Can influenza vaccination cause Guillain Barré syndrome?

Guillain Barré syndrome (GBS) is a rapidly developing, immune-mediated disorder of the peripheral nervous system that results in muscular weakness. Most people recover completely but some have chronic weakness. It can develop following a variety of infections, including influenza. In people who have been immunized with available vaccines, the frequency of GBS usually is the same as in unvaccinated people. Extensive studies and data analysis of influenza vaccines have only found a well established causal association with the 1976 vaccine that contained an H1N1 swine-influenza-like virus. No other clear association has been found with either seasonal or other pandemic influenza vaccines.

How can a repeat of the 1976 swine flu vaccine complications (Guillain-Barré syndrome) experienced in the United States of America be avoided?

During the 1976 influenza vaccination campaign, about 10 persons per million vaccinated persons developed GBS.

The reason why GBS developed in association with that specific vaccine has never been firmly established. The potential for the development of a similar risk with future vaccines can never be totally excluded. However, pandemic influenza vaccines are manufactured according to established standards, and are similar to recent well-studied influenza vaccines that have shown no association with GBS. Surveillance after vaccines have been sold (post-marketing surveillance) is being conducted to look for potential developments of serious adverse events. Safety monitoring systems are an integral part of strategies for the implementation of the new pandemic influenza vaccines.

(updated from 12 July 2009, 27 May 2009 and 2 May 2009 versions)

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