Our immune system is incredibly complex. It has many components that work together to protect us from pathogens (organisms that cause disease). The immune system has two main parts: innate immunity and adaptive immunity. Innate immunity, our first line of protection, is made of barriers to keep pathogens out of our body and other responses (like fever and inflammation) that stop pathogens from spreading. Adaptive immunity allows our bodies to recognize a pathogen, destroy it, and remember it. The memory allows us to respond more quickly the next time we encounter the pathogen and prevents us from getting sick. When we become infected with a pathogen and overcome it, we develop what is called natural immunity. Our bodies will create antibodies and memory cells in response to a pathogen even if we did not become sick.
Vaccines expose our immune system to all or parts of an inactivated pathogen. Our body recognizes it as foreign and creates antibodies and memory cells. If we are later exposed to this particular pathogen, we can mount a specific immune response and neutralize or kill it. Traditional vaccines present the inactivated or weakened pathogen to our immune system. The COVID-19 vaccines that are currently available in Canada stimulate our own cells to only produce spike protein component of the SARS-CoV-2 virus and present it to the immune system.
The spike protein is only one of 26 distinct proteins of the SARS-CoV-2 virus. As vaccine-induced immunity is only to the spike protein, it is narrow spectrum. This is a concern, because should a mutation occur in the spike protein, some of the spike-antibodies may no longer be able to strongly attach making them less effective in mounting an immune response. These are non-neutralizing or non-sterilizing antibodies. This means they will not prevent illness, also known as breakthrough infections, and may put people at risk of more severe disease through a process known as Antibody Dependent Enhancement.
Being exposed to the entire virus allows our body to make antibodies to all its proteins, not just the spike proteins. This means that natural immunity has a broad-spectrum, thus making it more effective against the variants. SARS-CoV-2 is just one of many coronaviruses and they share many equivalent proteins. Knowing this, it should not be surprising that scientists have learned that previous exposure to other coronaviruses, such as common cold viruses, provides immunity to SARS-CoV-2. Even those who were exposed to the original SARS almost 20 years ago showed immunity to SARS-CoV-2 in laboratory studies. In the vast majority of cases, natural infection prevents someone from getting sick from COVID-19 a second time.
One of the other important advantages of natural immunity is that the type of antibody response is more appropriate to the site of entry of SARS-CoV-2 as a respiratory virus. The secreted antibodies that are usually produced in the lung and airways in response to viral and bacterial are of the IgA, IgE and IgM classes. COVID-19 vaccines that involve intramuscular injections evoke the production of primarily IgG class antibodies, which are much less available in lungs and airways to engage the SARS-CoV-2 virus. This might account for why vaccinated people are still able to contract COVID-19 and transmit the virus.
Thus, natural immunity to COVID-19 has been found to be robust, appropriate, long lasting and complete.
