Influenza vaccinations: more than you wanted to know (but now I can say you have become an educated consumer of healthcare services)

I have had a number of people ask about the influenza vaccine in light of the current COVID-19 epidemic and thought a post about the whole influenza vaccine process would be helpful to explain.

Please sit back, take some time, and read (even re-read) leisurely as this is seemingly complicated but very understandable.

The history of global influenza monitoring and what goes into choosing a flu vaccine for the coming season

For many years there has been a department in the Centers for Disease Control (CDC) that constantly monitors influenza illness as it circulates around the world. They keep track of which strains are causing what illness in what location. They follow it in near real-time as these illnesses spread around the globe, coordinating with health agencies all over the world.

They have even done testing on innumerable individuals of all ages in the US over many years to learn who has what antibodies to which strain of the influenza virus and can identify which strains had passed through the US each year as far back as the 1950’s. (This in and of itself is frankly quite a remarkable feat!) From that information and ongoing testing, they can predict how much of the US is susceptible to getting influenza from this year’s strains coming through the US.

Based on extensive ongoing analysis of:

1. which strains are circulating around the world at the current time
2. combined with the huge amount of information on file about who has antibodies to which strain and
3. when those different strains last came through the US, then
4. the CDC predicts which strains of influenza are likely to come through the US in the coming November to April and how much of the general population of the US is susceptible to those strains.

The coming season’s vaccine contains those strains of the virus which are more likely to affect those in the US, NOT necessarily all those coming through the US this next flu season.

To illustrate this last point, let’s say the CDC predicts 7 strains are likely to come thru the US this coming season.

They see that 2 of the influenza strains came through in 2014 and 98% of us already appear to be immune to them (in essence, have antibodies against those two strains). Those strains would likely not be included in the vaccine.

Yet the other 5 influenza strains have not come through the US in any major way previously and only 5% of us are immune to them. Those strains would likely be included in this coming year’s vaccine, because they would make more people ill than the other 2, against which 98% of us carry antibodies.

In the end, each year’s influenza virus vaccine contains up to five (most years four) different strains of influenza virus, chosen by the CDC based on their calculations of 1) which ones are circulating around the world in the first half of the year and 2) which ones are most likely to affect us, based on our natural, or “herd”, immunity.

Vaccines that contain 4 strains are called quadrivalent vaccine. That just means 4 strains in the shot.

There are about 130 different strains of the influenza virus found in nature, so narrowing it down to 4-5 to include in the vaccine is quite a remarkable task.

Making each year’s flu vaccine

The process of how this coming year’s influenza vaccine is formulated and produced goes something like this:

1. The CDC decides in later June what virus strains are circulating around the world at that time and combine it with its huge database of the number of US citizens most likely to be affected by those strains. From that analysis they announce the strain composition for the coming season’s vaccine. (I actually get that information in the weekly publication from the CDC, but it makes no sense to me, so it comes in one ear and out the other. I just care that they have made their best guess possible.)
2. Vaccine manufacturers then go into warp drive and cultivate those strains of influenza virus in eggs. Because of the incubation medium, there is the caution about administering the vaccine to those allergic to eggs. From those virus cultures come the proliferation of viruses that make up the vaccine.
3. The vaccine manufacturers produce predominantly inactivated (dead) virus vaccines. For the vast majority of us the typical inactivated virus vaccine is recommended. That means the virus is broken up into pieces, none of which are contagious. You will not run the risk of getting influenza from an inactivated virus vaccine. (There are some who produce live virus vaccines, but these have very limited utility in adolescents or adults as they can potentially cause active influenza infections in those whose immune system is compromised.)
4. Shipping starts and delivery to large purchasers is fulfilled by mid-late August. (Hence the signs at Safeway, Walgreens, King Soopers, CVS, and others, “Flu vaccine available. Get yours now!”) More on this later in the article. The rest of us usually get our vaccine orders in mid-late September.

So what should you expect as benefit from the influenza vaccine?

IF you are exposed to a strain of influenza that is in the vaccine, you should expect a 90% chance of either not getting sick or having only a very minor illness. 10% of people will still get sick as if they were never given the vaccine.

IF you already have immunity to a strain of virus in the vaccine, then the vaccine would theoretically provide you no benefit. We do not at this time have the capability to easily detect for which of the 130 influenza strains you carry immunity, so answering the question whether you need this year’s flu vaccine cannot be addressed at present.

Why is there a yearly vaccine?

The induced immunity from the flu shot just doesn’t last long enough. The flu vaccine immunity only lasts about 6 months. It also takes 1 month after getting the vaccine to get full immunity.

If you get the flu vaccine today, you will not begin to become immune for about 2 weeks. So don’t get your flu shot today and run off to a flu-infested country and expect to be protected.

How long does immunity last if you got the influenza illness in the past?

The short answer is that it appears that immunity lasts for decades FOR THAT SPECIFIC STRAIN. (Remember there are 130 strains out there.) How many decades is not known, but probably at least several.

Let’s say you get the flu from strain 32. Can you develop immunity to other strains without having to get those infections? That is a term called cross immunity. We don’t know that answer for humans.

Some animal studies show some cross immunity to other strains. For instance a mouse who gets ill with strain 32, can also develop immunity against other strains without having been infected with those other strains.

How accurate is the prediction of what strains will come through the US and what actually does come through in a given flu season?

As a rough generalization, the CDC’s yearly vaccine predictions come out to be about:

1. 2 out of 5 years they are spot on, and the vaccine is highly effective, preventing a significant number of illnesses from those strains in the vaccine coming through the US that year.
2. 2 out of 5 years their predictions contain only 50% of the virus strains that actually come through the US. In those years the vaccine is somewhat protective across the population.
3. 1 out of 5 years the strains in the vaccine are completely NOT those virus strains that actually come through the US, and so the vaccine is basically not very effective.

Who should get the influenza vaccine and which one should you get?

Basically everyone should get the flu vaccine yearly. It is safe and very effective.

For those allergic to eggs a nasal spray vaccine is available.

If you are over age 65, you should get the high dose influenza vaccine. As we mature, our immune systems need a bigger jolt to induce appropriate immunity. Compared to others over age 65 who get the standard dose vaccine, those who get the high dose vaccine have an additional 20% reduction in likelihood of getting the flu.

There is no harm in individuals less than age 65 getting the high dose vaccine. We do not know at present whether the younger group gets the same benefit as those over age 65. That is only because the studies on the high dose vaccine only looked at those over age 65, who either got the standard vaccine or the high dose vaccine. Those under age 65 were just not included in the studies.

We only offer the high dose vaccine to everyone in my practice, regardless of age.

When should you get the flu vaccine?

Traditionally the Colorado flu season lasts from late November into mid-April. Given the limited duration of effectiveness of the vaccine, getting the flu shot around November 1 is optimal. Getting the flu shot in August, when the pharmacies get the vaccine, means that your immunity wears off in February, leaving you unprotected for the last half of the flu season. (Remember that the immunity only lasts about 6 months.)

Each year we see people who get the flu shot in August or September yet come down with the flu in the following March and April, because the vaccine immunity has worn off. I consider the early vaccination marketing by the pharmacies generally to be a public health disservice.

There is constant surveillance of influenza by the CDC, so if an early flu season appears to be imminent, then getting the vaccine sooner than November 1 would be recommended. I start getting regular updates weekly about cases of influenza in the US in September from the CDC. We would know promptly if there is an earlier than expected outbreak.

At a minimum I recommend waiting until October 1 to get the vaccine, and preferably later October to early November. We do not start giving the flu vaccine in the office until October 1, unless you are travelling to Europe, which tends to start the flu season a bit earlier than the US. My preference would be to have a long line of my patients come in on November 1 for their vaccinations, but the emotional and marketing pressure to get it sooner is just too hard to counter, so I compromise and start October 1 giving the vaccine.

Remember the 6 months of immunity. Getting your vaccine October 1st will get you immunity through the end of March, but not the tail end of the season in April. That is most but not all of the typical flu season.

What are the risks of getting the flu vaccine?

Prior to the mid-1980’s when manufacturing methods were radically altered, there were risks for some pretty serious complications, like the Guillain-Barre syndrome, from the old swine flu vaccines.

In the past 30+ years those complications have gone away because of newer purification techniques. Presently you may experience a day or so of aching, fatigue, feeling bad, and low grade fever after getting the vaccine. That rarely lasts more than 2 days. That should be the worst of it, and actually means that your immune system has reacted intensely to the vaccine. That’s a good sign.

There have been no severe complications (like Guillain-Barre syndrome) from the flu vaccine proven to be related to the vaccine in decades.

The bottom line

Get your yearly flu shot, regardless of your age. Only in years where there is a shortage of vaccine (like the year 30% of the eggs went bad) is there an age restriction.

Because of the irrational emotion associated with the COVID-19 pandemic, I suspect there will be a rush to get this coming year’s flu vaccine when it first becomes available. I find this unfortunate, as it will rapidly and prematurely deplete the supply, leaving many without the benefit of the vaccine in the later half of the flu season.

Getting the flu vaccine does not in any way prevent you from getting COVID-19.

Getting influenza AND COVID-19 would be a disaster for anyone. So don’t get either. Get your flu vaccine at a reasonable time (after October 1) and practice good social distancing, face mask use, and diligent hand sanitation.

Dr. Gipson