Influenza vaccines, also known as flu shots or flu jabs, are
vaccines that protect against infection by influenza viruses. New versions of
the vaccines are developed twice a year, as the influenza virus rapidly
changes. While their effectiveness varies from year to year, most provide
modest to high protection against influenza. The United States Centers for
Disease Control and Prevention (CDC) estimates that vaccination against
influenza reduces sickness, medical visits, hospitalizations, and deaths. When an immunized worker does catch the flu,
they return to work half a day sooner on average. Vaccine effectiveness in those under two
years old and those over 65 years old remains uncertain due to a lack of high-quality research. Vaccinating children
may protect those around them.
The World Health Organization (WHO) and the U.S. Centers for
Disease Control and Prevention (CDC) recommend yearly vaccination for nearly
all people over the age of six months, especially those at high risk. The European Centre for Disease Prevention
and Control (ECDC) also recommends yearly vaccination of high-risk groups. These groups include pregnant women, the
elderly, children between six months and five years of age, those with certain
health problems, and those who work in healthcare.
The vaccines are generally safe. Fever occurs in five to ten
percent of children vaccinated.
Temporary muscle pains or feelings of tiredness may occur as well. In certain years, the vaccine has been linked
to an increase in Guillain–Barré syndrome among older people at a rate of about
one case per million doses. Although
most influenza vaccines are produced using eggs, they are still recommended for
people who have severe egg allergies.
However, influenza vaccines are not recommended in those who have had a
severe allergy to a previous version of the vaccine itself. The vaccine comes
in inactive and weakened viral forms.
The live, weakened vaccine is generally not recommended in pregnant
women, children less than two years old, adults older than 50, or people with a
weakened immune system. Depending on the
type they can be injected into a muscle, sprayed into the nose, or injected
into the middle layer of the skin (intradermal). The intradermal vaccine was not available
during the 2018-2019 and 2019-2020 influenza seasons.
Vaccination against influenza began in the 1930s with large
scale availability in the United States beginning in 1945. It is on the World Health Organization's List
of Essential Medicines, the safest and most effective medicines needed in a
health system. The wholesale price in the developing world is about US$5.25 per
dose as of 2014. In the United States,
the vaccine costs less than US$25 per dose, as of 2015.
Medical uses
The Centers for Disease Control and Prevention (CDC)
recommends the flu vaccine as the best way to protect people against the flu
and prevent its spread. The flu vaccine can also reduce the severity of the flu
if a person contracts a strain that the vaccine did not contain. It takes about two weeks following
vaccination for protective antibodies to form.
A 2012 meta-analysis found that flu vaccination was
effective 67 percent of the time; the populations that benefited the most were
HIV-positive adults aged 18 to 55 (76 percent), healthy adults aged 18 to 46
(approximately 70 percent), and healthy children aged six to 24 months (66
percent).
Effectiveness
A vaccine is assessed by its efficacy – the extent to which
it reduces risk of disease under controlled conditions – and its effectiveness
– the observed reduction in risk after the vaccine is put into use. In the case of influenza, effectiveness is
expected to be lower than the efficacy because it is measured using the rates
of influenza-like illness, which is not always caused by influenza. Influenza vaccines generally show high
efficacy, as measured by the antibody production in animal models or vaccinated
people. However, studies on the
effectiveness of flu vaccines in the real world are difficult; vaccines may be
imperfectly matched, virus prevalence varies widely between years, and
influenza is often confused with other influenza-like illnesses. However, in most years (16 of the 19 years
before 2007), the flu vaccine strains have been a good match for the
circulating strains, and even a mismatched vaccine can often provide
cross-protection. The virus rapidly
changes due to antigenic drift, a slight mutation in the virus that causes a
new strain to arise.
Repeated annual influenza vaccination generally offer
consistent year-on-year protection against influenza. There is however
suggestive evidence that repeated vaccinations may cause a reduction in vaccine
effectiveness for certain influenza subtypes; this has no relevance to current
recommendations for yearly vaccinations but might influence future vaccination
policy. As of 2019, the CDC recommends a
yearly vaccine as most studies demonstrate overall effectiveness of annual
influenza vaccination.
Criticism
Tom Jefferson, who has led Cochrane Collaboration reviews of
flu vaccines, has called clinical evidence concerning flu vaccines
"rubbish" and has therefore declared them to be ineffective; he has
called for placebo-controlled randomized clinical trials, which most in the
field hold as unethical. His views on the efficacy of flu vaccines are rejected
by medical institutions including the CDC and the National Institutes of
Health, and by key figures in the field like Anthony Fauci.
Michael Osterholm, who led the Center for Infectious Disease
Research and Policy 2012 review on flu vaccines, recommended getting the
vaccine but criticized its promotion, saying, "We have overpromoted and
overhyped this vaccine...it does not protect as promoted. It's all a sales job:
it's all public relations".
Children
The CDC recommends that everyone except infants under the
age of six months should receive the seasonal influenza vaccine. Vaccination campaigns usually focus special
attention on people who are at high risk of serious complications if they catch
the flu, such as pregnant women, children under 59 months, the elderly, and
people with chronic illnesses or weakened immune systems, as well as those to
whom they are exposed, such as healthcare workers.
As the death rate is also high among infants who catch
influenza, the CDC and the WHO recommend that household contacts and caregivers
of infants be vaccinated to reduce the risk of passing an influenza infection
to the infant.
In children, vaccines again showed high efficacy, but low
effectiveness in preventing "flu-like illness". In children under the age of two, the data
are extremely limited, but vaccination appeared to confer no measurable
benefit. During the 2017–18 flu season, the CDC
director indicated that 85 percent of the children who died "likely will
not have been vaccinated."
Adults
In unvaccinated adults, 16% get symptoms similar to the flu,
while about 10% of vaccinated adults do.
Vaccination decreased confirmed cases of influenza from about 2.4% to
1.1%. No effect on hospitalization was
found.
In working adults, a review by the Cochrane Collaboration
found that vaccination resulted in a modest decrease in both influenza symptoms
and working days lost, without affecting transmission or influenza-related
complications. In healthy working
adults, influenza vaccines can provide moderate protection against
virologically confirmed influenza, though such protection is greatly reduced or
absent in some seasons.
In healthcare workers, a 2006 review found a net
benefit. Of the eighteen studies in this
review, only two also assessed the relationship of patient mortality relative
to staff influenza vaccine uptake; both found that higher rates of healthcare
worker vaccination correlated with reduced patient deaths. A 2014 review found benefits to patients when healthcare
workers were immunized, as supported by moderate evidence based in part on the
observed reduction in all-cause deaths in patients whose healthcare workers
were given immunization compared with comparison patients where the workers
were not offered vaccine.
Elderly
Evidence for an effect in adults over 65 years old is
unclear. Systematic reviews examining both randomized controlled and
case–control studies found a lack of high-quality evidence. Reviews of case–control studies found effects
against laboratory-confirmed influenza, pneumonia, and death among the community-dwelling
elderly.
The group most vulnerable to non-pandemic flu, the elderly,
benefits least from the vaccine. There are multiple reasons behind this steep
decline in vaccine efficacy, the most common of which are the declining
immunological function and frailty associated with advanced age. In a
non-pandemic year, a person in the United States aged 50–64 is nearly ten times
more likely to die an influenza-associated death than a younger person, and a
person over age 65 is over ten times more likely to die an influenza-associated
death than the 50–64 age group.
There is a high-dose flu vaccine specifically formulated to
provide a stronger immune response. Available
evidence indicates that vaccinating the elderly with the high-dose vaccine
leads to a stronger immune response against influenza than the regular-dose
vaccine.
A flu vaccine containing an adjuvant was approved by the
U.S. Food and Drug Administration (FDA) in November 2015, for use by adults
aged 65 years of age and older. The vaccine is marketed as Fluad in the U.S.
and was first available in the 2016–2017 flu season. The vaccine contains the
MF59C.1 adjuvant which is an oil-in-water emulsion of squalene oil. It is the
first adjuvanted seasonal flu vaccine marketed in the United States. It is not clear if there is a significant
benefit for the elderly to use a flu vaccine containing the MF59C.1 adjuvant. Per
Advisory Committee on Immunization Practices guidelines, Fluad can be used as
an alternative to other influenza vaccines approved for people 65 years and
older.
Vaccinating healthcare workers who work with elderly people
is recommended in many countries, with the goal of reducing influenza outbreaks
in this vulnerable population. While
there is no conclusive evidence from randomized clinical trials that
vaccinating healthcare workers helps protect elderly people from influenza,
there is tentative evidence of benefit.
Pregnancy
As well as protecting mother and child from the effects of
an influenza infection, the immunization of pregnant women tends to increase
their chances of experiencing a successful full-term pregnancy.
The trivalent inactivated influenza vaccine is protective in
pregnant women infected with HIV.
Safety
While side effects of the flu vaccine may occur, they are
usually minor. The flu vaccine can cause serious side effects, including an
allergic reaction, but this is rare. Furthermore, the common side effects and
risks are mild and temporary when compared to the risks and severe health
effects of the annual influenza epidemic. Flu vaccination may lead to side
effects such as a runny nose and a sore throat, which can last for up to
several days.
Guillain–Barré
syndrome
Although Guillain–Barré syndrome had been feared as a
complication of vaccination, the CDC states that most studies on modern
influenza vaccines have seen no link with Guillain–Barré. Infection with influenza virus itself
increases both the risk of death (up to 1 in 10,000) and increases the risk of
developing Guillain–Barré syndrome to a far higher level than the highest level
of suspected vaccine involvement (approximately 10 times higher by 2009
estimates).
Although one review gives an incidence of about one case of
Guillain–Barré per million vaccinations, a large study in China, covering close
to 100 million doses of vaccine against the 2009 H1N1 "swine" flu
found only eleven cases of Guillain–Barré syndrome, (0.1 per million doses)
total incidence in persons vaccinated, actually lower than the normal rate of
the disease in China, and no other notable side effects.
Egg allergy
Although most influenza vaccines are produced using
egg-based techniques, influenza vaccines are nonetheless still recommended for
people with egg allergies, even if severe. Studies examining the safety of influenza
vaccines in people with severe egg allergies found that anaphylaxis was very
rare, occurring in 1.3 cases per million doses given.
Monitoring for symptoms from vaccination is recommended in
those with more severe symptoms. A study of nearly 800 children with egg
allergy, including over 250 with previous anaphylactic reactions, had zero
systemic allergic reactions when given the live attenuated flu vaccine.
Other
Several studies have identified an increased incidence of
narcolepsy among recipients of the pandemic H1N1 influenza ASO3-adjuvanted
vaccine; efforts to identify a mechanism for this suggest that narcolepsy is
autoimmune, and that the ASO3-adjuvanted H1N1 vaccine may mimic hypocretin,
serving as a trigger.
Some injection-based flu vaccines intended for adults in the
United States contain thiomersal (also known as thimerosal), a mercury-based
preservative. Despite some controversy in the media, the World Health Organization's Global
Advisory Committee on Vaccine Safety has concluded that there is no evidence of
toxicity from thiomersal in vaccines and no reason on grounds of safety to
change to more-expensive single-dose administration.
Injection versus
nasal spray
Flu vaccines are available either as:
·
a trivalent or quadrivalent intramuscular
injection (IIV3, IIV4, or RIV4, that is, TIV or QIV), which contains the
inactivated form of the virus
·
a nasal spray of live attenuated influenza
vaccine (LAIV, Q/LAIV), which contains the live but attenuated (weakened) form
of the virus.
TIV or QIV induce protection after injection (typically
intramuscular, though subcutaneous and intradermal routes can also be
protective) based on an immune response to the antigens present on the
inactivated virus, while cold-adapted LAIV works by establishing infection in
the nasal passages.
Recommendations
Various public health organizations, including the World
Health Organization (WHO), have recommended that yearly influenza vaccination
be routinely offered, particularly to people at risk of complications of
influenza and those individuals who live with or care for high-risk
individuals, including:
·
the elderly (UK recommendation is those aged 65
or above)
·
people
with chronic lung diseases (asthma, COPD, etc.)
·
people with chronic heart diseases (congenital
heart disease, chronic heart failure, ischaemic heart disease)
·
people with chronic liver diseases (including
cirrhosis)
·
people with chronic kidney diseases (such as the
nephrotic syndrome)
·
people who have had their spleen removed or
whose spleen is not working properly
·
people who are immunosuppressed (people with
HIV, those receiving medications to suppress the immune system, and people on
chemotherapy) and their household contacts
·
people who live together in large numbers in an
environment where influenza can spread rapidly, such as prisons, nursing homes,
schools, and dormitories.
·
healthcare workers (both to prevent sickness and
to prevent spread to patients)
·
pregnant women. However, a 2009 review concluded
that there was insufficient evidence to recommend routine use of trivalent
influenza vaccine during the first trimester of pregnancy. Influenza vaccination during flu season is
part of recommendations for influenza vaccination of pregnant women in the
United States.
Both types of flu vaccines are contraindicated for those
with severe allergies to egg proteins and people with a history of
Guillain–Barré syndrome.
World Health
Organization
As of 2016, the World Health Organization (WHO) recommends
seasonal influenza vaccination for:
·
First priority:
·
Pregnant women
·
Second priority (in no particular order):
·
Children aged 6–59 months
·
Elderly
·
Individuals with specific chronic medical
conditions
·
Health-care workers
Canada
In 2008, the National Advisory Committee on Immunization,
the group that advises the Public Health Agency of Canada, recommended that
everyone aged two to 64 years be encouraged to receive annual influenza
vaccination, and those children between the age of six and 24 months, and their
household contacts, should be considered a high priority for the flu vaccine. The
NACI also recommends the flu vaccine for:
·
People at high risk of influenza-related
complications or hospitalization, including the morbidly obese, healthy
pregnant women, children six to 59 months, the elderly, aboriginals, and people
suffering from one of an itemized list of chronic health conditions
·
People capable of transmitting influenza to
those at high risk, including household contacts and healthcare workers
·
People who provide essential community services
·
Certain poultry workers
Europe
The European Centre for Disease Prevention and Control
(ECDC) recommends vaccinating the elderly as a priority, with a secondary
priority people with chronic medical conditions and healthcare workers.
The influenza vaccination strategy is generally that of
protecting vulnerable people, rather than limiting influenza circulation or
totally eliminating human influenza sickness. This is in contrast with the high
herd immunity strategies for other infectious diseases such as polio and
measles. This is also due in part to the
financial and logistics burden associated with the need of an annual injection.
United States
In the United States routine influenza vaccination is
recommended for all persons aged 6 months and over. It takes up to two weeks after vaccination for
sufficient antibodies to develop in the body.
The CDC recommends vaccination before the end of October, although it
considers getting a vaccine in January or even later to be still beneficial.
According to the CDC, the live attenuated virus (LAIV4)
(which comes in the form of the nasal spray in the U.S.) should be avoided by:
·
Children younger than two years
·
Adults 50 years and older
·
Concomitant aspirin- or salicylate-containing
therapy in children and adolescents
·
Children aged 2 through 4 years who have
received a diagnosis of asthma or whose parents or caregivers report that a
healthcare provider has told them during the past 12 months that their child
had wheezing or asthma or whose medical record indicates that a wheezing
episode has occurred within the past 12 months
·
Persons who are immunocompromised due to any
cause (including but not limited to medications and HIV infection)
·
Close contacts and caregivers of severely
immunocompromised persons who require a protected environment
·
Pregnant women
·
Persons
who have received influenza antiviral medications within the previous 48 hours
Within its blanket recommendation for general vaccination in
the United States, the CDC, which began recommending the influenza vaccine to
healthcare workers in 1981, emphasizes to clinicians the special urgency of
vaccination for members of certain vulnerable groups, and their caregivers:
·
Vaccination is especially important for people
at higher risk of serious influenza complications or people who live with or
care for people at higher risk for serious complications. In 2009, a new high-dose formulation of the
standard influenza vaccine was approved. The Fluzone High Dose is specifically
for people 65 and older; the difference is that it has four times the antigen
dose of the standard Fluzone.
The U.S. government requires hospitals to report worker
vaccination rates. Some U.S. states and hundreds of U.S. hospitals require
healthcare workers to either get vaccinations or wear masks during flu season.
These requirements occasionally engender union lawsuits on narrow collective
bargaining grounds, but proponents note that courts have generally endorsed
forced vaccination laws affecting the general population during disease
outbreaks.
Vaccination against influenza is especially considered
important for members of high-risk groups who would be likely to have
complications from influenza, for example pregnant women and children and
teenagers from six months to 18 years of age;
·
In raising the upper age limit to 18 years, the
aim is to reduce both the time children and parents lose from visits to
pediatricians and missing school and the need for antibiotics for complications
·
An added
benefit expected from the vaccination of children is a reduction in the number
of influenza cases among parents and other household members, and of possible
spread to the general community.
The CDC indicated that live attenuated influenza vaccine
(LAIV), also called the nasal spray vaccine, was not recommended for the
2016–2017 flu season in the United States.
Furthermore, the CDC recommends that healthcare personnel
who care for severely immunocompromised persons receive injections (TIV or QIV)
rather than LAIV.
Australia
The Australian Government recommends seasonal flu
vaccination for everyone over the age of 6 months. Australia uses inactivated vaccines. The flu vaccine is free for the following
people:
·
children aged 6 months to 5 years
·
people aged 65 years and over
·
Aboriginal and Torres Strait Islander people
aged 6 months and over
·
pregnant women
·
anyone over 6 months of age with medical
conditions such as severe asthma, lung disease or heart disease, low immunity
or diabetes that can lead to complications from influenza.
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