Medicine: Should I Get the H1N1 Vaccine?

By Dr. Mike Evans
Associate Professor,
Family & Community

University of Toronto;
Staff Physician, St. Michael’s Hospital*

Question #1: How likely is it that I will get sick from H1N1 this year?
Let’s start with the usual rates of flu. Typically your chances are about 9-12%, or about 9-12 of 100 people. This attack rate can go up to 42% if you have young kids.

H1N1 appears to have higher attack rates. The data last week from the US (they typically are a little ahead of us) show that they already at the highest rates in 5 years and it’s still early.


So, chances are still less than a coin toss that you’ll get H1N1 influenza, but it is on the rise, especially when you have kids.

Q2: How sick will I get?

Most of us get a cold seasonally. Definitely unpleasant but it is something we can cope with. Influenza is definitely more than the common cold.

H1N1 is really a version of influenza and unlike the common cold, symptoms include fever, muscle ache, intense fatigue, vomiting in some cases, stuffy nose and cough, wheezing, chills, shortness of breath….

Most people H1N1 flu is very unpleasant but they cope. Some people do get sick enough to be hospitalized. Reviewing US H1N1 data, about 11/1000 got sick enough from H1N1 to be hospitalized and 7/10,000 died from H1N1. 84 people have died from H1N1 so far in Canada (as of Oct 20, 2009) and this number will increase.


About 70% of people who have been hospitalized with H1N1 flu have had one or more medical conditions that placed them in the “high risk” category for serious seasonal flu-related complications. These include pregnancy, diabetes, heart disease, asthma and kidney disease.

It seems H1N1 affects younger people more often (combined, people 0-24 represent about 50% of cases). But, even though older people are less often afflicted, their rates of hospitalization are significant because they are more likely to be frail or have other diseases.

Q3: Will the H1N1 vaccine actually protect me?

Influenza vaccine is not perfect. The effectiveness depends on how the virus that is injected into you to make you immune (most are dead virus or a very small bit of live virus; H1N1 is dead) matches with the virus that is circulating in the community and on your own immune system. Older people tend to have less of a response. Usually the effectiveness rates for seasonal flu
vaccines is 70-90%.

It is still early but it looks like the match with what is in the community is very accurate and so hopefully the H1N1 vaccine will be on the upper end of effectiveness.

Typically we say that it takes 14 days for the vaccine to be effective. With H1N1 it looks like it works in about 10 days.

Q4: How do vaccines work?

Your body’s immune system is very smart. If you get invaded by a “bad virus”, your body
makes a “photocopy” and sends it around so that if the virus shows up again it neutralizes it
and you don’t get sick. Instead of allowing you to get sick, vaccines give you a dead or even a
very small live part of the virus so that your immune system is prepared to fight off the real
virus. The H1N1 vaccine gives you a dead virus for your immune system to “photocopy”.

Do vaccines weaken my immune system?

No. It makes it stronger. I often get asked if it isn’t better to get the immunity naturally? Yes and
no. If you get it “naturally” then you are immune but the problem is that you also get sick! So, for
example, if you have had documented H1N1 then you don’t need the vaccine (if H1N1 was
suspected but not tested for (which is common) then you still need the shot). The vaccine is just
an attempt to make it so you don’t have to get sick in the first place.

What’s an adjuvant?

The H1N1 vaccine in Canada has an adjuvant so it is important to discuss. An adjuvant is an additive that is sometimes put in vaccines to boost the effect. It “primes” the immune system so that a smaller amount of the vaccine has more effect. The advantage of this is that for many of my busy patients this means you only have to come in for one shot instead of two. The downside is that there isn’t as much trial experience with pregnant moms and very young kids. So, the vaccine isn’t recommended for kids under 6 months and, at this point, the recommendation is for pregnant women less than 20 weeks to go for the vaccine without the adjuvant.

Shots for tetanus and hepatitis have adjuvants.

Older People

Many older people (more than 60) are asking whether they should get the swine flu shot as they seem to be more “resistant”. The theory is that pre-1957 there was more swine flu around and
so people who were alive then have more resistance.

The H1N1 vaccine is recommended, even though the infection rate is lower, because your consequences are higher, if there are other underlying illnesses or disease and frailty.

Q5: What are the side-effects of the shot? Or … Didn’t I get sick last time I got it…?

Perception vs reality. About 20% of people will have symptoms after a flu shot- whether they get a fake/placebo or real flu shot. People often blame illness on the vaccine but sometimes they would have gotten sick regardless. Also the vaccines generally take two weeks to work so people can get sick with flu if exposed in that time.

People who should NOT get the flu shot are those with allergies to egg (the vaccine is grown on eggs) or those with a previous true allergic reaction to the flu shot. This is rare. People with sniffles can get the shot but if you have a high fever, it is not recommended.

Q6: What about these scary things I read on the internet about mercury and paralysis?

About 1 in a million will have a severe reaction (anaphylaxis or possibly Guillain-Barré Syndrome (GBS)

Data is conflicting as to whether a causal relationship exists between modern influenza
vaccines and GBS. If one exists, the risk is estimated to be very low (no more than 1 to 2 cases
per million doses). Since the introduction of universal influenza immunization in Ontario, there
has been no detectable increase in the number of new cases of GBS requiring hospitalization
at the population level. Just to be safe I would not give the vaccine to somebody where this condition was evolving.

Concerns about Thimerosal

Thimerosal is a preservative for vaccines that if we didn’t have in there we would have other
safety concerns. Most influenza vaccines available in Canada contain minute amounts of
thimerosal. Thimerosal has some mercury in it so it is rational to be concerned. Because of this
there has been large reviews of the safety of thimerosal. No studies have demonstrated an
association between thimerosal-containing vaccines and adverse outcomes. H1N1 has an
adjuvant which means we need about 1/10 of the usual amount of thimerosal. To keep things in
perspective, there is less mercury in the shot than in a tuna sandwich.

Q7: Does my decision affect others?

So far we’ve outlined a lot about your chances of getting sick but what I haven’t dealt with is the effect of your health on others.

I am biased because obviously, I see many older and/or sick people, so my main reason to get the
flu shot is to protect these people. If you get the shot you reduce the risk for people in your life who may struggle with dealing with H1N1 swine flu, such as those with other diseases and asthma.

Conversely, if you get the kids the shot, your chances of getting sick go down quite a bit.

Q8: Can you assess the risks vs benefits of getting the vaccine?

In a perfect world we would have large trials over years to reassure you (and me) that the vaccine is perfectly safe. But, if we waited for this, millions of people would get very sick. The H1N1 vaccine is like a new version of the flu shot that we have lots of experience with and an excellent safety record. As well, there are now enhanced reporting systems in North America to gather this data and there have been no concerning signals.

There are many things to consider, but my best guess looking at the data is that your chance of getting H1N1 flu and getting quite sick is likely about one in four. Higher if you have small kids.

The chance of a negative lasting consequence from the vaccine is around 1-2 in a million.

Your arm will likely be sore for two days.

Q9: What are the Public Health recommendations for the H1N1 vaccine?

Consult the Public Health Agency of Canada, but the summary is:
All Canadians 10 years of age and older should receive one dose of adjuvanted vaccine. Children from six months to nine years of age should receive the adjuvanted vaccine in two half-doses, administered at least 21 days apart.
Pregnant women should receive one dose of the unadjuvanted vaccine. If flu rates are rising in her area, women more than 20 weeks pregnant should be offered one dose of the adjuvanted vaccine.
All data to date indicates that adjuvanted vaccine is as safe as unadjuvated vaccine.

Do not get the vaccine if you know your are allergic to eggs or had an anaphylactic (severe allergic reaction) to any element of the vaccine, or if you previously experienced Guillan-Barré Syndrome within 8 weeks of receiving a seasonal flu vaccine.

The H1N1 flu vaccine is not approved for children under six months.

The H1N1 flu vaccine can be administered along with seasonal influenza immunization and other vaccines. Seasonal and H1N1 flu shots should be given in opposite arms. If an individual receives seasonal flu, H1N1 flu and pneumococcal vaccine in the same day, the seasonal flu shot and the pneumococcal vaccine should be given in one arm, and H1N1 flu vaccine in the other.

Q10: will you vaccinate your kids (and yourself)?


*Dr. Mike Evans is Associate Professor, Family & Community Medicine, University of Toronto; Staff Physician, St. Michael’s Hospital; Director, Health Design Lab; Director, Family Practice of the Future; Scientist, Li Ka Shing Knowledge Institute