flu shot

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OVERVIEW

Influenza vaccine is used to prevent influenza virus infection and its serious complications. Currently, the influenza vaccine is a Class II vaccine in most areas of China, and is a self-funded, voluntary vaccination.

Influenza viruses are categorized into four types: A, B, C, and D (or A, B, C, and D) according to their nucleoprotein and matrix protein. Influenza A viruses, in addition to infecting humans, are widespread in animals such as birds, pigs, horses, seals, and whales and mink. Influenza B is categorized into the Victoria and Yamagata lineages, which can be alternately prevalent in populations, and recent data indicate that seals can also be infected. Influenza C viruses infect humans, dogs, and swine and cause only disseminated cases of upper respiratory tract infections. Influenza D viruses, which mainly infect pigs and cattle, have not been found to infect humans. Currently, the viruses that cause seasonal epidemics are the H1N1 and H3N2 subtypes of type A and the Victoria and Yamagata lineages of type B viruses.

Influenza vaccines currently available in China include inactivated and live attenuated vaccines. According to the components contained in the vaccine, influenza vaccines include trivalent and quadrivalent. Trivalent influenza vaccine contains A (H3N2) subtype, A (H1N1) subtype and a lineage of B strains, a total of three components; tetravalent influenza vaccine contains A (H3N2) subtype, A (H1N1) subtype and a lineage of B Victoria and Yamagata, a total of four components.

Significance of Vaccination

Influenza vaccination is the most effective means of preventing influenza virus infection and significantly reduces the risk of influenza and serious complications in vaccinated individuals.

Influenza is usually characterized by acute onset of illness, fever (some cases may have a high fever of 39~40℃), accompanied by systemic symptoms such as chills, chills, headache, muscle and joint pains, extreme malaise, loss of appetite, and often a sore throat, cough, nasal congestion, runny nose, retrosternal discomfort, flushing of the face, and mild conjunctival congestion, as well as symptoms of vomiting, diarrhea, and so on. Severe patients may develop viral pneumonia, secondary bacterial pneumonia, acute respiratory distress syndrome, shock, diffuse intravascular coagulation, as well as extrapulmonary manifestations such as cardiovascular and neurological and a variety of complications.

Antibodies with protective levels are usually produced 2 to 4 weeks after influenza vaccination. When the body is re-exposed to the influenza virus of the viral type contained in the vaccine, it can stimulate the body to produce an immune response and play an immunoprotective role.

Vaccination Targets

According to the Technical Guidelines for Influenza Vaccination in China (2021-2022) issued by the Chinese Center for Disease Control and Prevention (CDC), the following groups of people are recommended to be prioritized for vaccination:

  • Medical personnel;
  • Participants of large events and security personnel;
  • Vulnerable people and employees of elderly institutions, long-term care institutions, welfare homes and other places where people gather;
  • People in key places;
  • Other high-risk groups for influenza, such as family members and caregivers including home-bound elderly aged 60 and above, children aged 6 months to 5 years, patients with specific chronic diseases, infants under 6 months of age, and pregnant women or women preparing to become pregnant during the influenza season.

    • Vaccination time

    The time of emergence and duration of peak influenza activity varies from place to place in China each year. In order to ensure that vaccinees are immunized before the high influenza season, it is recommended that localities arrange for vaccination as soon as possible after the vaccine starts to be supplied, and it is best to complete the immunization before the end of October.

    For those who have not been immunized by the end of October, immunization services are available throughout the pandemic season.

    Pregnant women can be immunized with inactivated influenza vaccine at any stage of pregnancy.

    Immunization procedures and doses

    Vaccination consists of intramuscular injection and intranasal spray.

  • Intramuscular injection: Adults and children over the age of 1 year are preferred to be vaccinated in the deltoid muscle of the upper arm; for infants and young children aged 6 months to 1 year, the best site is the anterolateral aspect of the thigh.
  • Intranasal spray: Trivalent live attenuated vaccine is administered by intranasal spray, and injection is strictly prohibited.

    • The number of doses is related to age and previous vaccinations.

  • Children 6 months to 8 years of age: Children 6 months to 8 years of age who received their first or fewer than 2 previous doses of influenza vaccine should receive 2 doses at intervals of ≥4 weeks; 1 dose is recommended for children who received 2 or more doses of influenza vaccine in 2020-2021 or previously.
  • Children and adults aged 9 years and older: 1 dose only.

    • Contraindications

    Hypersensitivity to any of the ingredients contained in the vaccine (including excipients, formaldehyde, cracking agents and antibiotics) is contraindicated.

    For people with mild to moderate acute illness with or without fever, it is recommended to receive the vaccine after the symptoms have subsided.

    The development of Guillain-Barré syndrome within 6 weeks after the last influenza vaccination is not a contraindication, but special attention should be paid.

    Special Tips:

  • Trivalent live attenuated vaccine (i.e. nasal spray type) is contraindicated for the following groups of people:
  • Pregnant women and children and adolescents treated with aspirin or medications containing salicylic acid components;
  • People at risk of cerebrospinal fluid leakage (via oropharynx, nasopharynx, nasal cavity, ears, etc.) (e.g., history of cochlear implantation, etc.);
  • People who have used oseltamivir and zanamivir 48h before vaccination, paramivir 5 days before vaccination, and baloxavir 17 days before vaccination due to the effect of antiviral drugs on the live virus in the vaccine, taking into account the metabolism time of different drugs.
  • Trivalent live attenuated vaccine (i.e., nasal spray) is not recommended for the following groups of people
  • Those who have taken influenza antiviral drugs 48 hours before vaccination;
  • Children aged 2 to 4 years with asthma;
  • People who are immunocompromised due to drug use, HIV infection, etc;
  • People who need to be in close contact with severely immunocompromised people.

    • Post-vaccination reactions and precautions

    The most common reaction to influenza vaccination is pain at the vaccination site, while symptoms such as generalized fever and fatigue are rare.

    Avoid vaccination on an empty stomach and observe for 30 minutes after vaccination.

    No vaccine or bacterial vaccine can provide 100% protection to the human body, so it is necessary to take precautionary and protective measures.