Influenza is an acute, contagious disease affecting the nose, throat and lungs, otherwise known as the respiratory tract. It is caused by a group of viruses, collectively known as the influenza virus. Although most cases of influenza are mild, it can lead to severe complications requiring hospitalisation or even death in more vulnerable populations. Fortunately, vaccines have been created, which are proven to reduce the risk of illness by 40-60% amongst the overall population. With annual reviews, new influenza vaccines are created to match the circulating influenza virus of the season.
How does Influenza spread
Direct/indirect contact via secretions:
- Kissing
- Sharing of utensils or food with an infected person
- Contact with virus-contaminated clothes, objects, hands of an infected person, followed by touching of your own eyes, nose or mouth
Airborne route via respiratory droplets:
- Talking, coughing or sneezing by an infected person can be inhaled by a healthy person resulting in the entry of the virus into the body.
Symptoms of Influenza
Symptoms of Influenza are similar to normal flu.
- Fever and chills
- Runny nose, sneezing
- Sore throat, coughing
- Headache
- Fatigue
Infected children can also experience diarrhoea, vomiting, and abdominal pain.
Global cases and fatality
According to WHO approximately 3 to 5 million people get infected with the influenza virus resulting in 250,000 to 500,000 deaths annually. In tropical and warm regions, influenza occurs year round. This is unlike temperate countries where there are “flu seasons”, typically occurring during the colder months of December to February.
Influenza has the potential to cause global pandemics just like COVID-19. Three other pandemics have been recorded in the twentieth century alone: Spanish Influenza in 1918, Asian Influenza in 1957, and Hong Kong in 1968. The Bird flu in 2004 and Swine flu in 2009 was also caused by the influenza virus.
Development of the Influenza Vaccine
There are four types of Influenza virus: A, B, C, and D. However, it is influenza viruses A and B that cause most human illness and are responsible for the flu season each year.
The first inactivated [containing viral particles] vaccine to be developed was a monovalent vaccine – a vaccine that would work against influenza virus A only. In 1942 a bivalent vaccine was developed after the discovery of the Influenza B virus.
In 1978 the first trivalent vaccine was produced. This trivalent vaccine contains two strains of Influenza virus A and one strain of Influenza virus B. There are currently two different strains of Influenza virus A (H1N1, H3N2) and B (B/Yamagata and B/Victoria) circulating. Hence, the World Health Organization recommends a quadrivalent vaccine i.e. a vaccine against these two strains of Influenza virus A and B.
The different types of influenza vaccines available
Currently, there are many vaccine options available against Influenza, e.g. live attenuated nasal spray influenza vaccine (LAIV), inactivated influenza vaccine (IIV), recombinant influenza vaccine (RIV) etc. Depending on the recipient’s age and health status, the most appropriate one will be chosen.
Live attenuated vaccine (LAIV):
Live attenuated vaccines are those in which the microorganism causing a disease loses its potential to cause disease, but retains a capacity for growth to induce an immune response within the inoculated host.
After a vaccine is given immune cells release antibodies in the circulation. This is called the humoral response. These antibodies then destroy the virus. Some cells of the immune system directly kill the virus-infected cells. This is called the cellular response.
This vaccine is given intra-nasally. It gives strong protection but may revert to its virulent form in some people whose immune systems are compromised. This vaccine is approved for people 2 through 49 years of age and should not be given to pregnant women and immunocompromised persons.
Killed or inactivated virus (IIV):
In this vaccine, the virus is inactivated by heat or chemicals. However, the portion which is recognised as “foreign” by another immune system is preserved. This vaccine is usually given in the form of injection. Its effectiveness as compared to LAIV is inconclusive.
Recombinant influenza vaccine (RIV):
Recombinant flu vaccine does not use the flu virus or chicken eggs in the manufacturing process. It contains recombinant proteins of the relevant four strains of influenza virus. These proteins function as the “foreign” material which induce the aforementioned immune response. It is a suitable alternative for those who have severe egg allergy.
Side effects of Influenza Vaccine
Like all other vaccines, the Influenza vaccine may cause adverse reactions in some people such as pain, swelling, and itching at the injection site. Severe allergic reactions though rare can happen and will require prompt medical attention. Despite the risk of side effects the benefits of receiving a vaccine easily outweigh its side effects.
Who should get vaccinated?
As the Influenza virus can be deadly for more vulnerable populations, annual vaccination is especially recommended for these high-risk groups e.g. older people (≥65 years old), young children (6 months to 5 years old), pregnant women, those with compromised immune systems due to medical conditions or medications, and those with chronic diseases e.g. asthma, diabetes, heart disease, kidney disease.
Vaccination should be done yearly, to provide optimal protection as a person’s immunity from vaccination wanes over time. Also because flu vaccines are updated from one season to the next due to constantly changing viruses, staying relevant with your influenza vaccine is important.
Featured image courtesy of the Smithsonian, National Museum of American History.
Article written in conjunction with World Immunisation Week 2021.
