Why do we need the flu vaccine and how does it elicit protection?
- julirestrepo546
- 14 hours ago
- 4 min read
Influenza viral infection causes the flu-
We have all had the flu and experienced the symptoms that come along with it, including fever, chills, coughing, runny nose, fatigue. Generally, within a few days we are able to recover, however, there are some individuals that are at high risk to whom the flu can be quite dangerous. Generally, we are accustomed to the fact that every year there are seasonal flu outbreaks and epidemics, however, there can also be flu pandemics that may pose much higher risk, hence why we require protective mechanisms like vaccines.
The flu results from infection from influenza virus. The influenza virus is a negative sense single stranded RNA virus that has 8 viral genome segments encoding for about 17 proteins. The virus can infect epithelial cells in the respiratory tract, which upon infection can results in cell death of infected cells. This impairs gas exchange, therefore causing respiratory distress, one of the dangers of the flu. As a defense mechanism, our immune system responds to viral infection by promoting infiltration of immune cells to site of infection, promoting inflammation in the lungs and respiratory tract. Recruitment of immune cells is essential for elimination of infected cells and clearance of the virus.
Why do we need annual flu vaccines?
Every year we are reminded to get our annual flu vaccines. Other vaccines, like MMR vaccines or the HPV vaccine you only get a few doses when you’re young and you are protected for the future. So why do we need a flu vaccine every year? Influenza virus can mutate very quickly, with every viral progeny coming out of an infected cells being a mutant. Therefore, as the virus spreads through an individual and as it spreads throughout the population it continues to mutate. This occurs because the genome replication machinery used by the virus does not detect errors in the process of its genetic replication, allowing for the virus to mutate.
What protective mechanisms does our body utilize to ensure protection against infection?
The purpose of vaccines is to provide immune memory that can protect us against future infection. In the case of influenza infection, antibodies can recognize the virus and prevent or reduce the severity of the infection. Antibodies can bind to part of the virus as a way to “tag” the virus so other cells can recognize the “tag” to detect the virus and remove it, therefore preventing further infection of other surrounding cells. Infected cells will also produce more virus that will continue to infect other cells, therefore antibodies can also "tag” infected cells so other immune cells can eliminate them before they produce more virus. Along with that, memory B cells are cells that can recognize the virus or its proteins, and upon recognition differentiate into cells that can secrete antibodies for protection. There are memory B cells that reside in the lungs, called resident memory B cells, that provide rapid local protection. These cells are established only when there is exposure via the respiratory tract, but because of their location, they can quickly recognize the virus during reinfection, allowing for a more immediate response.
Different types of Influenza vaccines
Inactivated Influenza vaccines- In this case, the virus undergoes chemical manipulation to take away its ability to replicate. Some may also undergo further treatment to remove parts of the virus leaving just the viral proteins, referred to as the split virus vaccine, to provide a boost to the immune response. The split virus vaccine is recommended for individuals that have previously been infected. In infants, who have not been infected, it would require that the split vaccine have an adjuvant, a chemical agent that will enhance the immune response to the vaccine, and therefore elicit better protection.
Live attenuated influenza vaccines- These are made from influenza viruses that are genetically modified to reduce their severity. These are administered intranasally, via the respiratory tracts like we would normally be infected, allowing for local exposure. This vaccine allows for establishment of resident memory B cells and more enhanced mucosal protection. It is recommended in individual age 2-49 years old, but not in pregnant individuals.
Recombinant vaccines- Do not depend on live virus, but rather use genetically engineered components of the virus to induce immune activation and protection. These vaccines can be produced faster and are useful in the event of a pandemic.
The goal is the universal flu vaccine
Currently flu vaccines require yearly updates, however a universal vaccine is in the works that would allow for long-lasting protection even as the virus continues to change and against multiple strains of influenza (Figure 1). So how does this work? There are parts of the virus, or segments of specific viral proteins, that are conserved. Conserved regions means that these sites of the viral proteins are not usually accumulating mutations and they are shared between different influenza strains. If our immune system could be geared to target these regions specifically, we could be protected for longer period of time and against multiple strains through one vaccine. While the universal vaccine is not available yet, there are multiple clinical trials testing multiple universal vaccines.
References:
1. Javanian, Mostafa et al. “A brief review of influenza virus infection.” Journal of medical virology vol. 93,8 (2021): 4638-4646. doi:10.1002/jmv.26990
2. Chen, Xiaoyong et al. “Host Immune Response to Influenza A Virus Infection.” Frontiers in immunology vol. 9 320. 5 Mar. 2018, doi:10.3389/fimmu.2018.00320
3. Mokalla, Vijay Reddy et al. “Influenza Vaccines: Current Status, Adjuvant Strategies, and Efficacy.” Vaccines vol. 13,9 962. 11 Sep. 2025, doi:10.3390/vaccines13090962
Assessed and Endorsed by the MedReport Medical Review Board