Flu Vaccine Effectiveness Over Time

Flu Vaccine Effectiveness Declines Around 10% Per Month Following Shot

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Influenza vaccine-provided immunity declines by around 10% each month following vaccination, according to study results published in Clinical Infectious Diseases. Researchers said September or October are the best months to get the shot.

An annual influenza vaccine is recommended for everyone 6 months and older in the United States. However, the best time to get an annual flu vaccine remains up for debate,Jill M. Ferdinands, PhD, an epidemiologist in the CDCs Influenza Division, told Healio. With mounting evidence showing that vaccine-induced immunity wanes over the course of the flu season, its important to consider whether early vaccination for example, in July or August may result in reduced immune protection against flu before the end of the season between March and May, especially among older adults.

According to Ferdinands, current recommendations attempt to balance the need to immunize the population before flu starts to circulate each year with concerns about waning immunity by recommending that vaccination be offered by the end of October.

This study hoped to gain a better understanding of the extent of in-season waning of influenza vaccine protection to help reduce uncertainty regarding the best time to get a flu vaccine, she said.

How Effective Is It

Flu vaccine effectiveness can change from year to year. At the time of writing this article, the 20202021 flu season is underway. In the coming months, scientists will begin to get estimates of the effectiveness of the 20202021 flu vaccine.

However, the does currently have information about the estimated effectiveness of the vaccines from past flu seasons.

The chart below shows the effectiveness of the flu vaccine for the previous five flu seasons.

When looking at vaccine effectiveness studies, its important to remember that the results can be influenced by several factors. These factors can include the population studied, the specific flu season, and how the study was conducted.

Study Period And Population

The study was conducted between September 1, 1998 and August 31, 2001, and included three consecutive influenza seasons, 1998/1999, 1999/2000 and 2000/2001, and the following off-seasons. The source population consisted of all residents of Stockholm County . The study population consisted of all individuals aged 65yrs during the period 19982001 . All such individuals were invited by post to receive, at reduced cost , influenza and pneumococcal vaccines over an 8-week period between September 1 and November 30 in each of the three seasons. The campaign was advertised on local television, in daily newspapers, on posters at general practitioners’ surgeries and at pharmacies.

For follow-up, each influenza season was defined as the period December 1 to April 30, and the off-season, when no influenza was likely to circulate, as the period May 1 to August 31. However, during the first study season , follow-up started on January 1, 1999 since outcome data for December 1998 were unavailable. During the time period in which vaccination was performed each year, the same person could initially belong to the nonvaccinated group and later to the vaccinated group, and this period was, therefore, not included in the primary outcome analysis.

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Single Virus Season And Age Group

For the baseline simulation mVE remained stable at 50% for the all-or-none mechanism but decreased to 42% for the variable mechanism and to 41% for the leaky mechanism . Variation in parameters from the baseline simulation showed that change in mVE was strongly related to seasonal infection rate, with minimal change when infection rate was 12.5% compared with a 3.9% to 4.5% change for an infection rate of 50% . Measured VE also declined more for heterogeneous exposures and higher values of VE and vaccine coverage. In all simulations, declines in mVE were greater for the leaky than for the variable mechanism.

Effect of parameters on absolute change in mVE per 30 days after influenza vaccination. Baseline simulation values are indicated with an asterisk . Infection rate indicates the cumulative percentage infected by end of season. Heterogeneity indicates population variability in numbers of potential exposures per day No indicates homogeneous exposure PM indicates POLYMOD distribution and EA indicates Eames distribution . Abbreviations: mVE, measured vaccine effectiveness VE, vaccine effectiveness.

Why Is Flu Vaccine Typically Less Effective Against Influenza A Viruses

There are a number of reasons why flu vaccine effectiveness against influenza A viruses may be lower.

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Predictors For Vaccination And Outcome

During the first study season, vaccination was more frequent among males than females, among those aged 7079yrs than those aged < 70 or > 79yrs, in married than in single subjects, and in those who had had white-collar jobs than in those with blue-collar jobs . All of these variables displayed confounding properties, i.e. were also associated with risk of death . There was a modestly higher proportion of vaccinated individuals among those with chronic cardiac or respiratory disease. The same pattern was also observed during the two following seasons .

Vaccinated-to-unvaccinated fully adjusted hazard ratio for various cause-specific mortalities during the three influenza seasons and following off-seasons : a) 1998/1999 b) 1999/2000 and c) 2000/2001. Horizontal bars represent 95% confidence intervals. All: all causes Res: respiratory Cir: circulatory Tum: tumours Ex: external.

What Is Herd Immunity

Vaccines dont prevent 100% of disease, and some individuals in a community dont get vaccinated. Yet entire communities can still be protected from a disease. This is the great thing about vaccines. If enough individuals are vaccinated against a disease in a community, everyone will be protected. This is called herd immunity.

The number of individuals who have to be vaccinated to reach herd immunity is different for every disease. Germs that are more contagious need a higher percentage of the population to be vaccinated. For example, herd immunity for measles takes between 85% to 95% of the population to be vaccinated. The remaining 5% to 15% who are not vaccinated will be protected because measles will not be spreading among the vaccinated population. For polio, 80% of the population needs to be vaccinated to reach herd immunity.

Herd immunity is very important for individuals who cannot get vaccinated. Newborns and individuals with weak immune systems cannot receive some vaccines, such as the live flu vaccine. They rely on everyone else in their community to get vaccinated, which protects them from getting sick. Because most vaccines do not prevent 100% of disease, herd immunity also protects people who have been vaccinated.

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Flu Vaccine Side Effects

Side effects of the flu vaccine are usually mild and go away without treatment in a few days. Common side effects include:

• Soreness, redness, and swelling where the shot was given
• Muscle aches

Rarely, allergic reactions occur, often within minutes of the vaccine being administered. If you experience swelling, racing heart or trouble breathing, seek medical attention immediately. Extremely rarely a person may develop Guillain-Barré syndrome, an autoimmune disease.

Besides Vaccination How Can People Protect Themselves Against The Flu

Getting a flu vaccine each year is the best way to prevent the flu. In addition to getting the flu shot, people should take the same everyday preventive actions to prevent the spread of flu, including covering coughs, washing hands often, and avoiding people who are sick. Antiviral drugs are an important second line of defense to treat the flu. These drugs are not a substitute for vaccination and must be prescribed by a health care provider.

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Who Shouldnt Get Vaccinated

If youre currently feeling sick, its best to wait until youre better.

Avoid the flu shot if you have a severe allergy to any of the ingredients that may be used in the vaccine, such as:

• egg protein
• monosodium glutamate , a stabilizer that keeps vaccines from losing their potency
• antibiotics, such as neomycin and gentamicin
• polysorbate 80, an emulsifier which keeps the ingredients from separating
• formaldehyde, which inactivates the flu virus

Babies under 6 months old shouldnt be vaccinated.

If youve had Guillain-Barré syndrome, talk to your doctor before getting the flu vaccine.

The nasal spray contains a weakened live virus. It shouldnt be taken by people who:

• are younger than 2 or older than 50 years
• are 2 to 4 years old and have asthma
• are 2 to 17 years old and take medications containing aspirin or salicylate
• are pregnant
• have life threatening allergies to the flu vaccine
• have a suppressed immune system
• are in close contact with someone with a suppressed immune system
• have taken antiviral drugs for the flu within the previous 48 hours

Talk to your doctor about the pros and cons of the nasal spray vaccine if you have:

• asthma or chronic lung disease
• a blood disorder

Study Highlights Challenge Of Evaluating Flu Vaccine

Other recent studies from the United States, Spain, and the United Kingdom have also shown waning interseasonal flu vaccine effectiveness, but experts warn that changing the timing recommendation is a complicated endeavor.

“My informal sense of the literature that the suggestion is strong enough that if people could reliably get vaccinated the week or two before the flu season starts, they’d be better protected,” Marc Lipsitch, PhD, a professor of epidemiology at Harvard University, told CIDRAP News. Lipsitch also penned a commentary on this study. “The more complicated thing is the trade-off between putting it off and not doing it at all,” he said.

Lipsitch said the public health risk of people failing to get vaccinated in the course of the year may be too great. In his commentary, he said a multitude of variables including, antigenic drift, and circulating strains, make studying vaccine timing a particularly challenging prospect.

“Better ways to evaluate flu vaccines are also urgently needed,” Lipsitch said in his commentary.

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Reasons To Consider Getting A Flu Vaccine

The flu vaccine significantly reduces the risk of contracting the flu. Consider the 20192020 season: People who had the vaccine were 39% less likely to get the flu than those who did not.

In addition to preventing illness, the flu vaccine also reduces the risk of hospitalization and death. Heres what researchers have found about the flu vaccine:

• Getting the vaccine reduces the risk of intensive-care unit hospitalization by 26% and the risk of death by 31%, according to a 2021 study.
• Among adults who are hospitalized for the flu, people who are vaccinated are 59% less likely to need intensive care unit care, according to a 2018 study.
• People with heart disease who get the vaccine are less likely to experience cardiac events.
• People with diabetes and chronic lung disease who get the vaccine are less likely to be hospitalized for those conditions.
• Pregnant people who get the vaccine are 40% less likely to be hospitalized for flu than pregnant people who did not get the vaccine.

In addition to protecting you, getting the flu vaccine can keep you from contracting the flu and passing it to others, including infants and the elderly, who are at higher risks for complications.

Pregnant people who get the vaccine help protect their baby from the flu , which can be valuable during the first six months of life when a child is not able to be vaccinated.

How Long Is It Effective For

Generally speaking, a flu vaccine should protect you through the current flu season. Youll need to receive another flu vaccine for the next flu season. There are several reasons why a flu vaccine is only effective for one flu season.

First, influenza viruses change constantly. Because of this, the strains included in this years flu vaccine may not be the most common strains during next years flu season.

Second, the level of protection generated by the flu vaccine wanes over time.

For example, a found that, across seven flu seasons, every additional 28 days after vaccination was associated with a 16 percent increase in the likelihood of testing positive for flu.

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Who Shouldn’t Be Vaccinated

If your child has a cold, itâs usually OK for them to get their shots on time. But if they’re very ill, the doctor may want to wait a while. Make sure the doctor knows if your child is or has been sick before they get a vaccine.

People with certain cancers and immune system problems should not get vaccines made with live viruses. These include the nasal spray flu vaccine , chickenpox , and MMR. Be sure your childâs doctor knows about all their health conditions.

If your child has had a severe allergic reaction to a vaccine in the past, they shouldnât get that shot again. They may also need to skip a vaccine if they have a severe allergy to:

The doctor can tell you whether or not a vaccine is right for your child.

Can Vaccine Effectiveness Be Different For Each Person

Scientists usually provide a vaccines effectiveness for a population. However, the effectiveness of a vaccine may be different for each individual. For example, age can impact how well a vaccine works. As we get older, our immune system may get weaker. A weaker immune system may not respond to a vaccine in a way that provides protection when the individual is exposed to the real germ. Other individual factors that may impact vaccine effectiveness include:

• Medications

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How Effective Are Flu Vaccines In Older Adults

In numerous studies since 2010, flu vaccines have helped protect older adults against influenza. Flu vaccination has reduced the risk of medically attended illness caused by A or influenza B viruses by more than 60% among people 65 years and older . Flu vaccines also have reduced the risk of flu hospitalization among adults 65 years and older due to A and influenza B. However, protection against influenza A viruses has been less consistent. On average, flu vaccines have reduced the risk of doctor visits for people with A flu by 24% and reduced the risk of hospitalization with A flu by 33% in adults 65 years and older . During seasons when the H3N2 vaccine component has been well-matched to the flu viruses circulating in the community, the benefit from flu vaccination has been higher. During these seasons, flu vaccine reduced the risk of hospitalization with A flu by 43% on average . But when the vaccine component was less similar to viruses in the community, the protection has dropped to 14% .For older adults, some studies have shown that some newer vaccines might be more effective than standard-dose inactivated vaccines without an adjuvant. Because flu viruses and effectiveness of flu vaccines can vary from one season to another, it isnt known whether any one of these vaccines will always be more effective in every flu season.

During Past Flu Seasons

The CDC tracks seasonal flu vaccine effectiveness each year. Over the past 17 years, the highest effectiveness was in the 2010-2011 season when it was 60% effective.

Here are the effectiveness rates over the past five seasons for which data are available:

• 20152016: 48%

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How We Track Effectiveness

The U.S. Flu Vaccine Effectiveness Networks began collecting data during the 2003-2004 flu season. Before then, how well the vaccine worked wasnât routinely monitored. The networks include three groups of hospitals and universities, each focused on a different part of the vaccineâs effectiveness.

At the University of Michigan School of Public Health, Joshua Petrie, PhD, is part of a team that works with the CDCâs U.S. Flu Vaccine Effectiveness Networks. The program recruits patients from outpatient clinics who come in because of a respiratory illness that might be related to the flu. Tests determine whether or not each patient has the flu, and researchers like Petrie compare the proportion that are vaccinated in those who tested positive to those who tested negative. âIf the vaccine works well, we expect a higher proportion of negative people to be vaccinated,â he says.

The data gathered by the various networks gets adjusted to account for differences in age, race, and medical conditions, then researchers determine the estimated effectiveness each year.

Who Should Have The Vaccine

In the 2022/23 flu season, the flu vaccine will be available for free to the following groups in England:

• All children aged 2 to 10 years on 31st Aug 2022
• Secondary school children in years 7, 8 and 9 – any remaining vaccine will be offered to children in years 10 and 11, subject to vaccine availability
• Those aged 50-64 years
• Those in long-term residential care homes
• Frontline health and social care workers
• Close contacts of immunocompromised individuals
• Those aged 6 months to 65 years in at-risk groups including people with the following health conditions:
• Respiratory diseases, including asthma
• Heart disease, kidney disease or liver disease
• Neurological conditions including learning disability
• A severely weakened immune system , a missing spleen, sickle cell anaemia or coeliac disease
• Being seriously overweight

Your doctor may recommend the flu vaccine in other circumstances as well.

Note that the eligibility criteria for the 2022/23 season are different to those in the 2021/22 season, so some people who were eligible for the flu vaccine last year may not be eligible this year.

Babies under 6 months old are too young to receive a flu vaccine. This is because they have maternal antibodies passed on from their mother which prevent the vaccine from working so well. Flu vaccination is offered to all pregnant women in the UK . As well as protecting pregnant women themselves, this also helps to protect their newborn babies from flu.

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