Q&A: Why the flu vaccine isn’t always effective

A doctor from Brigham and Women’s Hospital spells out what you need to know for the 2017-2018 flu season.

Flu Vaccine Effectiveness
A pharmacist handles a syringe for the flu vaccine in the consultation room at his dispensary on October 6, 2017 in downtown Bordeaux, France. Georges Gobet / AFP / Getty Images

Have you gotten your flu shot yet? Flu season started earlier than usual in Massachusetts this year, prompting the state’s public health officials to urge residents to get the flu vaccine to prevent spreading of the illness.

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The advisories were made on the heels of medical experts expressing concern that the vaccine for the 2017-2018 season won’t be effective against the dominant strain of the virus in United States. In the New England Journal of Medicine last week, doctors pointed to “mounting concern” over the record level of flu in Australia, where the vaccine was only 10 percent effective against the dominant strain H3N2. The American flu vaccine uses the same composition, indicating possible low vaccine effectiveness and a similarly severe flu season.

As we prepare for a potentially severe influenza season, we must consider whether our current vaccines can be improved and whether longer-term, transformative vaccine approaches are needed to minimize influenza-related morbidity and mortality,” the doctors wrote.


To learn more about how the vaccine is created and why it may not be effective in 2017, we spoke with Paul Sax, the clinical director of the Division of Infectious Diseases at Brigham and Women’s Hospital. 

This interview has been edited for clarity and length. How is the flu vaccine created?

Paul Sax: The way the flu vaccine is chosen is that there are surveillance sites collecting samples from people who have the flu around the world. And then they submit their information to a central panel organized by the WHO. Then, based on their best predictions, they say that next year’s flu vaccine should contain these strains because these are the circulating strains that we’re most likely to see. That’s one component.

The other is that most flu vaccines in the world are made in hen eggs. The process basically is that you try to create the flu virus that’s circulating. Then you kill that virus, and the inactivated virus is incorporated into the vaccine.

What causes a vaccine to be ineffective?

One of the most humbling aspects of epidemiology is predicting flu seasons. Even the people who know the most about influenza have learned to be humble about it, because they often get predictions wrong.


Sometimes the circulating strain that they predict is going to happen doesn’t. And then another one emerges, so there’s a mismatch between what’s in the vaccine and what’s circulating. That’s one problem.

The second problem we’ve just learned about is that the very process of making the vaccine introduces, for some reason, an error into the vaccine that makes it less antigenic, which means it doesn’t stimulate the right antibody response or a strong antibody response. The current theory is that’s what the problem is with the current H3N2 mismatch. It’s not that we got the H3N2 wrong, it’s that the way that we make the vaccine somehow makes it less effective against the H3N2.

Is H3N2 the predominant strain of flu this year?

So far it’s been mostly H3N2 in the United States. And that was the dominant strain in Australia. They have a flu season during our summer, so there have been people who say we’re going to see something very similar to what happened in Australia. However, I think an important caveat is that even that information doesn’t 100 percent ensure that we’re going to have an epidemic similar to Australia’s. Because we actually had H3N2 last year. So it may be that we have a different sort of set up for immunity or partial protection.


We just don’t know yet how it’s going to play out.

How bad is the flu season expected to be this year?

That is the million-dollar question.

It is true that if you compare flu activity this year to last year and the year before, we are seeing more influenza at this stage in early December than we did in those years. But — and this is an important ‘but’ — just because you see flu activity early does not mean that it’s going to be necessarily a bad flu season.

Sometimes you get a lot of the activity out of the way early and then it turns out to be a regular, or even a light, year. So far it’s more than last year, but really until the end of the season, which doesn’t happen until early spring, we really don’t know.

Why has it gotten off to an earlier start this year?

One theory is that H3N2 typically causes more severe disease, especially in the elderly. And as a result, because it’s an H3N2 dominant season so far, these are people who are seeking care for influenza like illness. That’s one theory.

But again. We don’t know whether it’s going to be a bad flu season overall. It’s something we learn when we look back at the season, not something when we look forward.

When will we know whether this year’s vaccine is working or not?

That is always something people are interested in hearing but we can’t really say until after the season is completed. The way that that’s done is they look at people who either have laboratory confirmed influenza or influenza-like illness and compare the rates for people who received the vaccine against people who did not.


It does appear from some studies that even people who get the vaccine where the vaccine isn’t 100 percent effective, it does appear to attenuate — meaning lessen the severity — of the illness.

Flu Vaccine Effectiveness

Vials of flu vaccines in Philadelphia in 2013.

How effective should the vaccine be if it’s properly matched?

There are several studies that tell us that the flu vaccine really isn’t one of our most effective vaccines, if you say effective means that everybody who gets it is 100 percent protected. Estimates that I tell my patients is that in a typical year, the flu vaccine may be 60 percent effective or so. Some would say that maybe my estimate is a little high but I think that’s fair.

The influenza vaccine in Australia last season against this particular isolative H3N2 was only 10 percent effective. So that’s clearly lower than normal. But that doesn’t mean it was 10 percent effective for the other strains that was circulated. It was about 60 percent effective for influenza B for example.

That’s just a way of saying that it varies from strain to strain, and I think 10 percent is better than zero percent. Also, if you think that the person next to you in the crowded subway might be coughing and have the flu — he or she might have influenza B and then you would be protected. I think overall this is still the best way we have over preventing influenza, so I’m still recommending the vaccine.


What else can people do to prevent getting the flu?

The kind of things that I’m going to recommend are going to sound very common sense. Clearly flu is spread by aerosols. It’s spread by people who sneeze and cough, and it’s spread on surfaces. So all of the things that are recommended for people to prevent the flu are very common sense. You don’t want to have close contact with people who are sick. You want to not go out yourself into public when you’re sick. It’s really better to stay home from school and work if you have the flu.

When you sneeze, when you cough, you should cover your mouth and your nose. You should clean your hands frequently — I think this is something we all need to be reminded of. And something that some people say is you should try and keep your hands away from your mouth and nose as much as possible because that’s how viruses can be spread from you hands to your mouth and your nose.

There is evidence that good habits can help you avoid the flu.

The 100-year anniversary of the flu pandemic of 1918 is approaching. Could a flu pandemic happen today?

There is definitely a concern among public health officials, which is one reason why there’s such a global collaboration to study flu activity. Something like what happened in 1918 could happen again if the flu strain that’s circulating undergoes a major shift in its antigens and therefore we’re not protected at all. And, this is important, if it also becomes very transmissible.


So far some of these very unusual flu strains that we have no immunity to are not actually that transmissible from human to human, so we have not had a flu pandemic. But I can assure you that it’s high on the list of international public health concerns and is something that’s being watched very carefully.

Is there anything else you want to say?

One thing I want to stress is that sometimes people believe the flu vaccine gives them the flu. And that really can’t happen. The flu vaccine contains an inactivated or killed virus. And second, people who say that may be mistaking other viral respiratory infections for influenza. So they might get their flu vaccine and then a couple of days later get a very bad cold.

Influenza and bad colds have very similar symptoms, but they are not identical. A flu vaccine was not the cause of the cold. The cause of the cold is that they came into contact with someone else who had a bad cold. And since this is the season where you start to see more respiratory infections in general because people are inside more, there’s less fresh air, there are more colds in the season when people get the flu vaccine. So that’s probably the cause of the misconception.