Two recent experiments have confirmed that individuals previously infected with SARS-CoV-2, the virus that causes COVID-19 virus, can be reinfected.
Interestingly, the two reinfected individuals from the study had different results. During the second infection, the person in China had no symptoms, while the person in the US experienced a more serious form of the disease. Therefore, it is unknown if the SARS-CoV-2 immune response would secure us against subsequent reinfection.
Does that mean a vaccine would fail to protect against the virus as well? Definitely not. Firstly, there is still uncertainty as to how frequent these reinfections are. More importantly, as seen in the US patient, a fading immune response to natural infection doesn’t mean that we can’t produce an effective, safe vaccine.
How does our body react to an infection?
Initially, any infection in our body triggers a non-specific innate immune response in which inflammation is caused by white blood cells. This could be enough for most of the viruses to clear up.
But the adaptive immune system is activated in more severe infections. Here, T and B cells (who has the task of responding to infections) identify distinct virus-derived structures (or antigens). Infected cells are identified and destroyed by T cells, while B cells develop antibodies that neutralize the virus.
This adaptive immune response is delayed during a primary infection. Primary infection refers to the first time a person is infected with a specific virus. It takes a few days before immune cells that recognize the particular virus are triggered and expanded to contain the infection.
What does a vaccine do?
This primary infection is imitated by a vaccine, which provides antigens (the toxic substance from the virus) that prepare our immune system and produces memory cells that can be triggered quickly if and when we get infected by the virus in the future.
So, the vaccine is a way of voluntarily introducing the virus into us so that antibodies are developed and our body is ready to battle the virus when we are actually infected with it later on.
Long after the infection is cured, some of these T and B cells, called memory cells, survive. It is these memory cells that are essential for long-term protection. The memory cells get triggered quickly in a subsequent infection by the same virus and induce a robust and accurate response to block the infection.
Now, a common doubt may arouse whether the substance that we are injecting into our body through the vaccine can cause serious infection. No, there is little chance of serious infection as the antigens in the vaccine are extracted from weakened or noninfectious material from the virus.
Benefits of a vaccine shot
There are other benefits of vaccines over natural infections. They can be designed, for one thing, to target the immune system against particular antigens that elicit better responses.
For example, the vaccine for human papillomavirus (HPV), which is a sexually transmitted disease, induces a stronger immune response than the virus infection itself. One explanation for this is that the vaccine contains high amounts of a viral coat protein, more than what would occur in a normal infection. This causes antibodies to be strongly neutralized, making the vaccine very effective in avoiding the infection.
Natural immunity against HPV is particularly poor, as different strategies are used by the virus to escape the host immune system. Many viruses, including HPV, have proteins that block the immune response or simply lie low to avoid detection.
A vaccine’s immunogenicity, i.e. how effective it is in generating an immune response, can also be fine tuned. Usually, agents called adjuvants kick-start the immune response and can increase the immunogenicity of vaccines.
In addition, to promote adequate immune responses in the right places, the dosage and route of administration can be controlled. Traditionally, except for respiratory viruses such as measles, vaccines are given by injection into the muscle. The vaccine produces such a powerful response in this case that antibodies and immune cells reach even the surfaces of the mucosa in the nose.
However, the effectiveness of the oral polio vaccine has been due to a localized immune response in the gut, where poliovirus replicates. Similarly, delivering of the coronavirus vaccine directly to the nose may lead to increased mucosal immunity in the nose and lungs, providing protection at the point of entry.
Our natural immune system
An effective vaccine that improves the body’s natural immunity allows us to first understand our natural immune response to the virus. SARS-CoV-2 neutralizing antibodies have been observed to be active in our body for up to four months after infection.
Previous studies have indicated that antibodies usually last for a couple of years against coronavirus related infections. Fortunately, decreasing levels of antibodies do not always mean that our immune responses against the coronavirus is weakening.
More promisingly, a recent study showed that T memory cells activated SARS response in an individual almost two decades after he was infected. SARS or Severe Acute Respiratory Syndrome was a contagious and sometimes fatal respiratory illness which appeared in 2002 in China. Though it was quickly contained, the disease managed to spread globally in a few months.
So, out of the approximately 320 vaccines being developed against COVID-19 worldwide, the answer to long-lasting immunity could be one that favors a good T cell response.
