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Mild side effects from flu shots are common; very serious adverse effects are very rare

Posted on:  2020-10-05

Key takeaway

Flu vaccines are updated each year based on the circulating strain of the virus and undergo clinical trials to ensure their safety. Mild, short-lived side effects from flu shots are common, such as pain at the site of injection, soreness and fever. These side effects result from the intended immune response stimulated by the vaccine. Serious adverse reactions to flu shots are very rare and can be associated with allergic reactions to specific ingredients in the vaccine.The benefits provided by the flu vaccine largely outweigh the risks for the population. Flu vaccination is particularly important this year given the risks of coinfection by the flu and COVID-19.

Reviewed content

Lacks context

Flu shots lead to severe or life-threatening conditions; flu shots are unsafe

Source: Facebook, Facebook users, 2020-10-01

Verdict detail

Inadequate support:: Scientific evidence does not support the claim that serious or life-threatening conditions commonly occur after a flu shot. On the contrary, the flu vaccine has a strong track record of safety.

Full Claim

Flu shots lead to severe or life-threatening conditions; flu shots are unsafe


Claims that the flu vaccine for the 2020 – 2021 season caused dangerous side effects started circulating on social media in early October 2020 (see here). For example, some social media users reported developing an abnormally high fever after receiving the flu shot, and others claimed that someone who received a flu vaccine nearly died.

The 2020-2021 flu season starts in October in the Northern Hemisphere, and flu shots are currently being distributed. Flu vaccination is particularly important in 2020 because health systems strained from the COVID-19 pandemic might not be able to cope with an additional influx of patients experiencing flu complications. Furthermore, cases of coinfection with COVID-19 and the flu may produce worse disease outcomes in patients compared to single infections with either the flu or COVID-19[1]. Consequently, ensuring the safety of the flu vaccine is crucial to encourage the public to receive vaccines and achieve a vaccine coverage of the population in accordance with the national target of 70%.

The flu vaccines currently available are licensed by the U.S. Food and Drug Administration (FDA) based on safety testing and clinical trials prior to release. Every year between February and March, the FDA selects influenza strains to be included in the vaccine, based on recommendations by the World Health Organization and other experts. These recommendations are based on predictions of which types of influenza virus would circulate during the flu season. Every year, the new vaccines aiming at protecting against the selected virus strains are then produced and tested for efficacy and safety before FDA approval.

Given that the flu season just started in the Northern Hemisphere, it is not possible to draw conclusions about the prevalence of mild or serious adverse effects of flu shots in the general population. However, scientific evidence from previous years demonstrates a strong track record of safety for the flu vaccine, contrary to claims that flu shots are dangerous.

Posts from numerous social media users reporting serious or life-threatening adverse reactions to the flu shot offer no evidence to support their claims. Instead, dangerous reactions from the flu vaccine are very rare according to the U.S. Centers for Disease Control and Prevention (CDC). Mild side effects to the flu vaccine are common and short-lived, as indicated by the CDC. The main side effects are soreness, headache, fever and muscle ache, which result from the body’s immune response stimulated by the flu vaccine.

Vaccination works by using a harmless antigen such as an inactivated virus to trigger the immune system to mount an immune response against that specific virus. This immune response leads to the formation of an immunological memory that allows a faster and stronger response in the event of a subsequent encounter with the pathogenic form of the virus. In other words, some immune reaction from the body is necessary for a vaccine to effectively immunize a person.

An immune response triggered by a vaccine typically involves initial inflammation resulting from the activation of the body’s defense mechanisms. Inflammation is characterized by swelling, redness, and soreness. Therefore, it is normal for the flu vaccine to induce these side effects, which are transient. For instance, clinical trials of Afluria Quadrivalent, the most commonly used flu vaccine for the 2020 – 2021 flu season, indicate that 47% of 18-64 years old patients reported pain at the site of injection and 21% reported headaches. Fever was reported by only 2% of patients.

Similarly, the CDC reports that 65% of people vaccinated for the flu experienced pain at the injection site during the first week after vaccination, which usually did not interfere with the person’s typical activities. According to an article by the Advisory Committee on Immunization Practices from the CDC, placebo-controlled trials also found that the most common side effect was soreness at the site of injection, which lasted less than two days. In addition, a review of the safety profiles of influenza vaccines showed that mild and transient side effects, such as local pain and fever, are common[2]. Therefore, it is unsurprising to experience pain, muscle aches or even a fever in the first few days after receiving a flu vaccine.

The rate of systemic symptoms is the same between people receiving a flu vaccine and people receiving a placebo. Serious adverse effects from flu shots are very rare, as they were reported in less than 1% of cases during clinical trials, according to the CDC. In some cases, these adverse reactions are due to a patient having an allergic reaction to some components of the vaccine, like the protein albumin, also known as egg white protein. This immediate hypersensitivity occurs at a rate of one per one million vaccine injections and is not a new side effect for the 2020 – 2021 flu season[3]. Although life-threatening allergic reactions to flu vaccines are possible, they are very unlikely to occur, contrary to what these claims suggest.

Because common side effects to flu vaccines are mild and transient, it is safer for most people to get a flu shot than to get the flu. During the 2019 – 2020 flu season, the CDC estimated that 7.52 million illnesses, 3.69 million medical visits, 105,000 hospitalizations, and 6,300 deaths were averted due to flu vaccinations. These estimates are based on the coverage and effectiveness of flu vaccines each year. In addition to the benefit at the population level, vaccination decreases the risk of developing a severe form of the disease. The CDC reports that vaccinated people who get infected nonetheless have a lower risk of needing hospitalization compared to unvaccinated people. Similarly, a study in New Zealand demonstrated that vaccination decreased the risk of admission into intensive care units because of flu complications[4].

In summary, claims alleging that serious, life-threatening reactions to the flu shot are common during the 2020 – 2021 flu season are unsupported by scientific evidence. It is too early to draw conclusions about the effectiveness and frequency of adverse effects from the flu vaccine for the 2020 – 2021. Contrary to this claim, data from previous years indicate that mild and transient side effects from flu shots, such as soreness, are common and an indication of the body responding effectively to the vaccine.

Furthermore, serious adverse events to the flu vaccine are extremely rare. Overall, the benefits of the flu vaccine greatly outweigh its risks. Considering the burden averted by flu vaccination each year and the current risk of coinfection of flu and COVID-19, the flu shot remains a safe and efficient strategy to protect the population.


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