Andrew Pekosz, an associate professor of molecular microbiology and immunology at Johns Hopkins University’s Bloomberg School of Public Health in Baltimore, answers some questions about the safety of the H1N1 vaccine.

Q: Is the new vaccine safe?

All the data from the ongoing clinical trials of the 2009 H1N1 vaccine indicate it is just as safe as the regular seasonal flu (vaccine). This was expected, since the 2009 H1N1 vaccine is made in exactly the same way as the seasonal flu vaccine.

Q. Why did the previous swine flu vaccine cause Guillain-Barre syndrome?

A. A complex question! First, it’s not clear how much of an increased risk of Guillain-Barre syndrome was associated with influenza vaccination, partly because it’s not really certain how many cases of GBS occur annually in the United States. Most rigorous studies have suggested there was either a low or no risk of developing GBS after immunization with the 1976 H1N1 vaccine when compared to the general population. GBS has not been convincingly or consistently linked to any other influenza vaccine before or since. So the link between GBS and influenza vaccines remains controversial, but it is a relatively low risk, if any.

Q. The swine flu seems mild compared with seasonal flu. So why expose my family to a new vaccine? Wouldn’t it be better just to get the new flu?

A. A couple of things. The disease caused by the 2009 H1N1 is very consistent with the disease caused by seasonal flu. We also know that most of the population under the age of 60 is susceptible to this virus, and in fact, the majority of people over 60 are susceptible as well. We have never seen as many cases of flu as we have seen this summer, indicating that the virus can spread fairly well under conditions where seasonal flu doesn’t spread well. So all these things together cause us to be very concerned about a huge increase in the number of flu cases in the coming winter. The vaccine looks safe and effective and is our best way of cutting down the total number of flu cases, thereby reducing the severe cases of flu as well.

Q. Will you get the vaccine?

A. I have already received my seasonal flu vaccine (FluMist, by the way). I plan on getting the 2009 H1N1 vaccine, but I don’t fall into any of the high-risk groups for that vaccine so I won’t be getting it until later in the vaccine campaign.

Q. Have there been examples of vaccines that seem safe in clinical trials and then cause problems in the general population?

A. If a problem with a vaccine emerges after it has completed clinical trials, it usually involves a rare side effect that only became obvious when a large number of people receive the vaccine. There are a number of independent groups that will be tracking people who received the 2009 H1N1 vaccine to look for just such types of rare adverse side effects.

Q. How do doctors know if apparent side-effects are linked to the vaccine or not?

A. Patients who get immunized are asked to report back if they experience any of a number of symptoms that are associated with side effects of a vaccine. The number of cases of these symptoms in the vaccinated group is then compared to the number of cases that occurs in the general population.