What You Need to Know About the Flu Vaccine: What It Is, How It Works and Potential Side Effects to Watch Out For
What You Need to Know About the Flu Vaccine: What It Is, How It Works and Potential Side Effects to Watch Out For.
Every year in late summer and early fall, you’re hit from all angles with dire warnings that if you don’t get a flu vaccination, you’re bound to get sick and possibly die. From social media to billboards, TV, radio and print ads, to posters in grocery stores and pharmacies, there literally is no place you can go to escape the push. The pressure is even more intense in your own health care provider’s office.
What’s more, some states have now made flu vaccinations mandatory for child care and school admittance, while most health care centers make the vaccination mandatory for work. And, no matter where you hear or see it, the message is always the same: The only way to stay well and protect yourself and those around you is to get vaccinated.
It’s the U.S. Centers for Disease Control and Prevention (CDC) that is behind all this, through its Advisory Committee on Immunization Practices (ACIP)1 — and their recommendation is that everyone age 6 months and older should get a flu vaccination. This advisory has been in place since 2010.2
Unlike drug ads, however, you never hear or read any warnings about possible side effects of a flu vaccine, and that’s concerning, considering there is mounting evidence that these adverse effects could be worse than the disease itself. Read on to learn more about what this vaccine is, what ingredients are in it and what possible adverse reactions you need to watch out for.
Hopefully, the insights you gain can help you make an informed choice regarding influenza vaccinations for you and your family.
What Is Influenza?
Influenza (flu) is a contagious viral respiratory disease that can be mild or severe. It infects the nose, throat and, sometimes, lungs. In severe cases, particularly when someone is immunocompromised or has comorbid (simultaneous) chronic diseases, influenza can be fatal. According to the CDC, symptoms of influenza can include:3
- Sore throat
- Muscle, body aches
- Runny or stuffy nose
- Extreme fatigue
If you have flu, you can infect others anywhere from the day before your symptoms begin to five to seven days after you get sick — or even longer, depending on your age and immune system. Usually the worst period for infecting someone else is within the first three or four days.4
The virus is spread by breathing flu droplets in the air that are spread when a person with flu coughs, sneezes or talks, or by touching surfaces where the droplets have landed, and then touching your eyes, nose or mouth. The CDC notes that an infected person can infect others up to 6 feet away.
Complications of flu can include anything from minor ear or sinus infections to serious problems such as myocarditis, encephalitis and myositis, as well as life-threatening respiratory conditions like pneumonia, or even kidney failure, any of which can lead to death.
What Is a Flu Vaccine?
An influenza vaccine, aka flu shot, is a vaccine that’s routinely recommended before and during a particular “flu season” with the intention of preventing influenza. There are four types of influenza viruses, identified with the letters A, B, C and D.5
The individual A and B viruses are then broken down into subtypes and strains based on proteins and other characteristics, such as lineage and strains, of each virus. Using these viruses and subtypes, world health officials choose which ones will be included in each year’s flu vaccines.
When determining which viruses to use in flu vaccines for the U.S., the CDC, together with the World Health Organization, monitors flu activity in the Southern Hemisphere. Then, judging from which viruses are prevalent there, the U.S. Food and Drug Administration (FDA) decides in February which viruses and subtypes will be in the current vaccines in the U.S.6
After you get the vaccine, the antigens (viruses) and adjuvants (components added to the vaccine to enhance the antigens’ response) work together to stimulate antibodies which are supposed to provide protection against the viruses in the vaccine. Generally, it takes about two weeks after getting a flu vaccination for this to happen.7
What Are the Types of Flu Vaccines?
There are three types of flu vaccines. Two are given via injection; one is a mist sprayed into the nostrils:8
- Inactivated influenza vaccine (IIV)
- Recombinant influenza vaccine (RIV)
- Live attenuated influenza vaccine (LAIV), which is a mist
These vaccines are either trivalent with three influenza viruses in them, or quadravalent with four different viruses in them. The following viruses will be in the trivalent vaccines for the 2018-2019 flu season:9
- Influenza A H1N1 (Michigan strain)
- Influenza A H3N2 (Hong Kong strain)
- Influenza B (Brisbane strain)
The quadrivalent vaccines will contain all of the above three, plus influenza B (Phuket strain). Examples of these types of vaccines include:10
|Trivalent Flu Shots||Quadrivalent Flu Shots|
|Standard-dose trivalent shots (IIV3) that may be different for various age groups||Quadrivalent flu shots for people of different ages, even babies as young as 6 months and children as young as 4 years old|
|High-dose trivalent shot or Fluzone High-Dose Seasonal Influenza vaccine11 recommended for people 65 years old and above||Intradermal quadrivalent flu shot or the Intradermal Influenza Vaccination12approved for people between 18 and 64 years old|
|Egg-free recombinant vaccine or the Flublok Seasonal Influenza (Flu) Vaccine13 for people 18 years old and above, including pregnant women||Recombinant quadrivalent flu shot or Flublok Seasonal Influenza (Flu) Vaccine,14for people 18 years old and above, and pregnant women.|
|Vaccine with an adjuvant15 also recommended to people 65 years old and above||Live attenuated influenza vaccine, a mist sprayed into the nostrils|
Note: The live virus nasal spray called FluMist was withdrawn during the 2017-2018 season because it yielded such a high failure rate (97 percent) during previous flu seasons.16 The CDC has now reapproved it for non-pregnant individuals ages 2 years to 49 years for 2018-2019.17 However, the American Academy of Pediatrics (AAP) in September 2018 said that while they support both vaccines, they recommend that children get the shot rather than the mist.18
Flu Viruses Aren’t the Only Things in a Flu Vaccine
Besides flu viruses, flu vaccines have other ingredients in them that are present in varying degrees, from trace amounts to measurable quantities. Some of these ingredients are “adjuvants,” which are added to hyperstimulate your body’s response to the vaccines’ antigens (viruses), with the intention of accelerating the production of antibodies to the targeted viruses.19
The most common flu vaccine ingredients are listed below. While the CDC and vaccine makers say these ingredients are necessary to make the vaccine safe and effective,20, be aware that all them are also poisonous, carcinogenic or potentially harmful to your skin and gastrointestinal, pulmonary, immune and neurological systems. They also can be allergenic in some individuals:21,22
1.Antibiotics — To prevent bacteria formation during production and storage, manufacturers add antibiotics such as gentamicin or neomycin.
2.Formaldehyde — As a flu vaccine ingredient, formaldehyde is used to deactivate and decontaminate the flu viruses and toxins in the vaccine.
3.Chicken egg proteins — Historically, most flu viruses have been grown in fertilized chicken eggs, as this environment allows viruses to grow and reproduce. Viruses are separated from the egg and added to the vaccine after completing development, with some traces of chicken egg proteins being transferred. As such, people with an egg protein allergy should rethink getting a flu shot.
4.Canine (dog) kidney cells — Instead of being grown in chicken eggs, two vaccines, Flucelvax and Flucelvax Quadrivalent, are grown in a canine kidney cell line and inactivated with a detergent called cetyltrimethylammonium bromide.
According to the manufacturer, these vaccines do not contain preservatives or antibiotics, but they may contain “residual amounts” of the canine cell protein, the bromide, polysorbate 80 (a surfactant and emulsifier) and a sterilizer called β- propiolactone.23
5.Gelatin: — Pork-based gelatin acts as a stabilizer for the purpose of helping to maintain the flu vaccine’s effectiveness from production to use, and shields the vaccine from harmful heat- or freeze-drying effects.
6.Thimerosal — Thimerosal is a preservative that contains approximately 50 percent mercury. While it’s no longer found in most pediatric vaccines, it’s used in multidose vials of flu vaccines to help prevent contamination by bacteria, fungi or other germs as the vial is repeatedly used.30
Should I Be Worried About Mercury in Flu Shots?
The mercury in thimerosal is ethylmercury, which is not the same as the methylmercury found in fish. When it enters the body, thimerosal breaks down to ethylmercury and thiosalicylate.31 While the CDC insists that thimerosal is safe in flu shots because it clears quickly from the body, be cautious about any form of mercury, as it’s a neurotoxin.32
Even the state of California recognizes thimerosal as a reproductive toxicant that has been found to cause severe mental retardation or malformations in the offspring of mothers who were exposed to it while pregnant — and that wasn’t the only concern California had.33
Additionally, a case control study featured in a 2017 Brain Injury article revealed that mercury exposure from thimerosal-containing vaccines is linked to a high risk for emotional disturbance. Study authors came to this conclusion after looking at information from the Vaccine Safety Datalink (VSD) database.34
In his article “Excessive Vaccination and Autism,” neurosurgeon Dr. Russell L. Blaylock also emphasizes that mercury is a known immunosuppressant and stimulates your microglia (immune cells in the brain) , despite a typical flu vaccine having very little amounts of it. Plus, increasing the quantities of mercury delivered to the body via a flu shot may:35
- •Cause excitotoxicity since mercury delays glutamate removal from extracellular surfaces
- Poison antioxidant enzymes like catalase, glutathione peroxidase and SOD needed for brain cell production
- Reduce glutathione levels drastically via some mechanisms
- Trigger brain damage by disrupting energy production
•Disturb your neurons’ energy production capabilities, since its mitochondria absorb more mercury compared to other cells, thereby increasing their sensitivity to excitotoxicity
Should I Get a Flu Vaccination?
As mentioned earlier, the CDC and ACIP recommend that everyone age 6 months and over get an influenza vaccination. However, there have been reports of people feeling sick after a flu shot and experiencing side effects (more on these later),36 so if you are thinking of getting a flu shot, you should weigh the possible risks against the presumed benefits before getting a flu vaccination. According to MedicalNewsToday, some groups of people who should be extra cautious of, and possibly avoid, a flu vaccine include:37,38
|Children under 6 months old|
|People with severe allergies to gelatin, antibiotics or eggs|
|People who already developed a severe allergic reaction to a previous flu vaccine|
|Those with severe, life-threatening allergies to any ingredient in the vaccine|
|People previously diagnosed with Guillain-Barré Syndrome|
|People who don’t feel completely healthy|
How Long Does the Flu Vaccine Last?
Flu vaccines are supposed to be effective for one flu season (around a year),39 although there are instances when antibodies against the seasonal inactivated flu vaccine decrease only a few weeks or months after vaccination. The flu vaccine’s reported effectiveness may also vary, depending on age and risk groups.40 For example, immunocompromised individuals41 and persons aged 65 and older42 have lower immune responses to the vaccine.
This is why the CDC recommends seniors take a “high dose vaccine” with four times the antigen, with the adjuvant MF59 (squalene) in it, in an attempt to boost the body’s response to the vaccine.42 It is also why they say children age 6 months through 8 years require two doses if they are getting vaccinated for the first time.44
Do Flu Vaccines Work?
Despite claims regarding its protective benefits, the flu vaccine is not as effective as health officials want you to think. Every year the CDC evaluates the previous season’s flu activity and how well the vaccine worked — or didn’t. The CDC’s conclusion for the 2017-2018 season was that the vaccine’s overall adjusted effectiveness was an abysmal 36 percent. Broken down by virus strain, the estimated effectiveness was:45
- 25 percent effective against the A(H3N2) virus
- 67 percent effective against A(H1N1)pdm09 virus
- 42 percent effective against influenza B viruses
In 2015, a CDC analysis46 revealed that, between 2005 and 2015, the flu vaccine was LESS than 50 percent effective more than half the time, so the 2017-2018 numbers really were no great surprise. For the 2018-2019 season, the CDC says the vaccines have been updated to “better match circulating viruses,” so they are hopeful the numbers will be better; however, they admit that previous studies have shown the vaccine is still usually only 40 percent to 60 percent effective.47
A study released by Rice University, however, is less optimistic. Looking at 6,610 human flu sequences, researchers predict that the current year’s flu vaccine will be no more effective against the dominating A virus than the previous year’s — in other words, 20 percent.48
Furthermore, results from a 2014 meta-analysis conducted by the Cochrane Collaboration showed that vaccines may only deliver a slight protective effect against the flu and influenza-like illnesses (ILI, with moderate-certainty evidence), since 71 people have to be vaccinated in order to prevent just one flu case, and 29 people need to be vaccinated to prevent one ILI case.49
Additionally, a 2017 study conducted by French researchers proved that flu vaccines were ineffective at significantly decreasing influenza symptoms. The findings, which were published in the journal Vaccine, involved checking data among vaccinated and unvaccinated elderly patients with the flu. Results showed that the vaccinated patients only complained of less headaches — quite lackluster for a vaccine that claims to provide all-out protection against this disease.50
Is the Flu Vaccine Safe?
The CDC states that in most cases the flu vaccine is safe for everyone aged 6 months and older, with “rare exceptions” for certain people.51 However, there are numerous studies showing there are risks to getting an influenza vaccine. For example:
- International Journal of Epidemiology (2006) — Senior citizens who received a flu shot were 44 percent less likely to die during flu season, but were 61 percent MORE likely to die before after their vaccination, before the season even began.52
- Clinical Infectious Diseases (2010) — Results from this study, which focused on a Canadian community, revealed that people given the seasonal flu vaccine had a higher risk for a pandemic H1N1 flu infection.53
- Clinical Infectious Diseases (2014) — People were more likely to experience reduced protection against the disease if they received vaccinations for consecutive flu seasons. Researchers at the B.C. Centre for Disease Control initially thought that vaccines from the 2008 flu season may help lower people’s risk against the H1N1 virus, but were shocked to find out that these nearly doubled a person’s infection risk.54,55
- Vaccine (2017) — This study found that pregnant women who had received a pH1N1-containing flu shot two years in a row were more likely to suffer a miscarriage within the following 28 days.56
Also, you need look no further than the data contained in the U.S. National Vaccine Injury Compensation Program (NVICP), which has been set up to compensate people who experience severe side effects from vaccines, to see that the influenza vaccine is the most compensated one on the list.57
Can You Get Sick From the Flu Shot?
The flu vaccine is described as the best tool to stay healthy during the flu season, but sometimes vaccinated persons will say the flu shot made them sick. Is this possible? In a word, yes. It has to do with the way your immune system works. A term known as heterologous immunity describes the memory your immune system has of previous infections, which allows it to respond to similar infections later. But this “protection” goes two ways. As noted in the Journal of Leukocyte Biology:58
“Immunity to previously encountered viruses can alter responses to unrelated pathogens. This phenomenon, which is known as heterologous immunity, has been well-established in animal model systems. Heterologous immunity appears to be relatively common and may be beneficial by boosting protective responses. However, heterologous reactivity can also result in servere immunopathology.
In other words, your flu vaccination may backfire in that it could make you more susceptible not only to flu in a coming year, but to other illnesses right after you get the vaccine, including respiratory diseases. In fact, research presented at the 105th International Conference of the American Thoracic Society revealed that children who get seasonal flu vaccinations were three times more at risk of hospitalization than children who do not — and the risk was even higher for children with asthma.59
What Are the 12 Side Effects of the Flu Shot?
As of October 31, 2017, more than 148,088 flu shot-related reports were made to the federal Vaccine Adverse Events Reporting System (VAERS). Out of these reports, there were 1,399 related deaths, 11,008 hospitalizations and 2,802 related disabilities.60 Some of the flu shot’s short-term side effects include:61,62
- Low-grade fevers
- Sore throat
- Aches and muscle pains
- Soreness, redness or swelling where vaccine was given
- Itchiness and toughness where the vaccine was given (for people given an intradermal flu shot)
- Nasal congestion
Some people may also have severe allergic reactions to the flu shot.63 These may appear a few minutes or hours after administration of the vaccine. If you can, watch out for the following allergy symptoms that may develop after a flu shot:64,65
- Swelling in the face, particularly around the eyes or lips
- Breathing difficulties
- Weakness or dizziness
If you’re wondering how long these side effects last, it’s often for one to two days.66 However, there are long-term side effects of the flu shot that may cause adverse health conditions and pain for increased periods of time.
An example is shoulder injury related to vaccine administration (SIRVA), one of two recent vaccine reactions added to the NVICP Vaccine Injury Table (VIT).67 Around 202 people were compensated by the Vaccine Injury Compensation Program in 2016 for SIRVA caused by different vaccines.68
Experiencing shoulder pain after a flu shot is quite common,69 alongside other reactions such as nerve damage, frozen shoulder and rotator cuff tears. The U.K.’s NHS notes as well that you may experience a sore arm after getting a flu shot.70
ABC Action News journalist Ashley Glass learned about these side effects the hard way. After an annual flu shot in October 2017, she experienced difficulty moving her arm. In her Facebook post detailing the incident, commenters were quick to point out that she may be experiencing a flu shot side effect.71
As mentioned below in the children’s side effects section, the flu vaccine may increase your seizure risk. The CDC notes that a higher risk of fever-caused seizures is possible after a child is given a flu shot with the pneumococcal vaccine (PCV13) and/or DTaP vaccine. To prevent this, inform your doctor before the flu vaccination if you have a child who has experienced a seizure.72
Lastly, the flu vaccine may cause extensive damage to your body’s nervous system, and predispose you to the following conditions:73
•Brain and nerve disorders: Flu shots have been linked to brain inflammation and encephalopathy, Acute Disseminated Encephalomyelitis (ADEM), optic neuritis, partial facial paralysis, brachial plexus neuropathy and vasculitis.
•Guillain-Barré Syndrome (GBS):74 A flu shot can slightly raise your risk for GBS, with estimates being one or two cases per 1 million vaccinations.75 GBS, which can develop two to four weeks after the vaccination, is an autoimmune disorder that may target your peripheral nervous system, trigger temporary or permanent paralysis and other chronic health problems, or even cause death.
Common GBS symptoms to watch out for include muscle weakness, numbness, an unsteady gait, tingling, pain, or facial or limb paralysis.76
If you experience any of the flu vaccine’s side effects , consult your doctor immediately to address them and prevent debilitating flu vaccine reactions.
Why You Should Avoid Getting a Flu Shot While Pregnant
The CDC recommends all pregnant women to get a flu shot (not the live virus mist) during their pregnancy. Despite the CDC’s recommendations, there are known risks linked to getting a flu shot while pregnant or breastfeeding, such as fainting, headaches, fever, muscle aches, nausea, fatigue, and soreness, redness or swelling at the injection site.
There are also women who may have a flu shot allergy . If you know that you have a high risk for severe allergic reactions, avoid this vaccine at all costs.77 Plus, there are studies that have highlighted the potential dangers of getting a flu shot during pregnancy:
•Human & Experimental Toxicology (2012) — Researchers discovered an “ascertainment-corrected rate of 590 fetal-loss reports per 1 million pregnant women (or 1 per 1,695) who were vaccinated.” They came to this conclusion after analyzing reports from the Vaccine Adverse Events Reporting System (VAERS) for three flu seasons.78
•Vaccine (2017) — As mentioned earlier, receiving a flu shot when pregnant may raise the risk for miscarriage within 28 days. This particular study examined women administered a pH1N1-containing flu shot during consecutive flu seasons (2010 to 2011 and 2011 to 2012). Overall, 485 women had miscarriages in their first or second trimester, with 17 of them being vaccinated for two years in a row. According to Amanda Cohn, the CDC’s adviser for vaccines:79,80
“I think it’s really important for women to understand that this is a possible link, and it is a possible link that needs to be studied and needs to be looked at over more [flu] seasons. We need to understand if it’s the flu vaccine, or is this a group of women [who received flu vaccines] who were also more likely to have miscarriages.”
Flu Shot Side Effects in Babies, Toddlers and Children
A child may develop aches, low-grade fever, redness or soreness, or rashes from a flu shot.81 Other allergic reactions to the flu vaccine include:82
- Breathing difficulties
- Rapid heartbeat
Children administered the flu nasal spray may experience the following too:85
- Runny nose
- Muscle aches
Research has also highlighted that flu vaccinations may increase children’s risk for illnesses:
- Flu-related hospitalization — A 2009 Science Daily article details that children between 6 months and 18 years of age who received annual flu shots were more likely to be hospitalized because of flu-related illnesses, with this risk already rising among children with asthma.86
- Pandemic H1N1 or swine flu — Results published in PLoS Med in 201087 showed that people who received the trivalent flu vaccine in the 2008-2009 flu season were more likely to be diagnosed with pandemic H1N1 “swine flu” the year after.88,89
- Respiratory infection — In a 2012 Clinical Infectious Diseases Study article, authors noted that children given the trivalent flu vaccine were four times more likely to be affected with a respiratory infection.90
There Are Better Ways to Protect Yourself From the Flu
Whether your doctor recommends that you get a flu shot or another vaccine altogether, you need to understand that a vaccine is a medical procedure with risks. Therefore, you should make it a practice to always learn the risks of any vaccine, including the flu vaccine, alongside the benefits that your doctor or the CDC say you will get from it.
This is called “informed consent” and it is your basic human right to know all the risks of a vaccine or any medical procedure you undergo, and not just whatever benefits you’re told you’re going to get.
Extra effort may be needed if you’re a parent, since children, who aren’t aware of the ramifications associated with vaccines, may be herded in to a mass vaccination program at their school — even when you may have objected to it in writing. Furthermore, some states have already begun implementing measures that may curtail opportunities for people to make decisions for themselves.
For instance, a New York resolution passed in June 2018 gave the state’s health department the right to require flu vaccines for preschoolers.91 Meanwhile, a bill passed in California has already eliminated the personal belief vaccine exemption for families living on welfare, stripping away their legal rights to avoid vaccines altogether.92
If you want to be protected from influenza, consider practicing immune system-boosting strategies that may help shield your body against flu-causing agents. Optimizing vitamin D levels,93,94,95 increasing vitamins B1 and C intake96 or taking zinc lozenges when initial flu symptoms appear are known to work better than vaccines in addressing the disease before it worsens.
Frequently Asked Questions (FAQ) About Flu Shots
Q: Do flu shots hurt?
A: Yes. Common flu shot side effects include soreness, redness or swelling where the vaccine was given. In rare cases, patients may find it difficult to move their arm, or will notice that their shoulder or arm hurts after receiving a flu shot.
Q: Why do flu shots hurt?
A: University of Pittsburgh professor and PittVax director Dr. Richard Zimmerman emphasizes that the pain you get after a flu shot is linked to an inflammation response your body experiences after being vaccinated.
Small quantities of proteins called antigens from the vaccine are released into your body so your white blood cells know the structure of the flu virus. Once the vaccine is administered, your immune system immediately senses the viruses and starts “attacking” them. The end result is inflammation and pain.97
Q: Are flu shots bad?
A: Not only do flu shots cause side effects or allergic reactions, but some studies have discovered that they may increase your risk for pandemic H1N1 flu infection, Guillain-Barre syndrome or even seizures.
Q: Is there mercury in flu vaccines?
A: Yes. Thimerosal, an ethylmercury-containing compound, is a vaccine ingredient. It plays a role in preserving the life of multidose flu vaccines and aids in reducing contamination risk caused by germs, bacteria or fungi.
Q: Can you get a flu shot while pregnant?
A: Although the CDC recommends that pregnant women should get a flu shot, take note that it can come with mild to severe effects. Receiving a flu shot may lead to nausea , fatigue, headaches, fever and muscle aches, allergic reactions or even miscarriage.
Q: Is the flu shot a live virus?
A: The shot is not a live virus, but the flu mist is. As the CDC reiterates, flu shots administered through needles only contain flu viruses that have been inactivated or killed.
Q: Can you still get the flu after you get a flu vaccine?
A: You can get the flu even if you receive a flu shot. One reason is because the flu vaccine is notoriously not effective. In the 2017-2018 flu season, it was only 36 percent effective.
Also, according to Medical News Today, even if the vaccine protects you against the antigens in it, you may still become infected with a flu virus that isn’t present in the vaccine, or if you develop flu before the vaccine takes effect. Another reason why the flu vaccine can make you sick is linked to a concept called heterologous immunity.
This means that while the immune system was signaled to prepare itself against the three or four viruses in the vaccine, it won’t be able to shield itself against other flu viruses or non-flu respiratory infections, raising your risk for virus-, bacteria- or other pathogen-caused infections. Either way, if you notice flu symptoms after receiving a flu shot, consult your doctor right away.
Q: Can you get a headache after a flu vaccine?
A: Yes. Headaches are one of the side effects linked to a flu shot.
Q: Can you get hives after a flu vaccine?
A: Hives can develop in people who may be allergic to flu shots or any of the ingredients in the vaccine.
Q: Can you get diarrhea after a flu vaccine?
A: Not really. While the flu virus can trigger diarrhea and seizures, especially in children,98 there isn’t enough research linking the flu vaccine to diarrhea.
Q: Can you get a sore throat after a flu vaccine?
A: Yes. According to the National Vaccine Information Center, a sore throat is one of the moderate side effects that may develop after receiving a flu shot.
Q: How long does the flu vaccine last?
A: A typical flu vaccine is supposed to be effective for one flu season (roughly around one year). Take note, however, that this is a general estimate since many factors can affect a flu vaccine’s effectiveness, such as age and risk groups. Furthermore, the efficacy of the vaccine may lessen within only a few weeks or months after the vaccination.