Vaccines are a marvel of medicine. Few interventions can be credited with saving so many lives.
But it may surprise you to learn that not all vaccines provide the same level of protection. Some prevent you from contracting symptoms of the disease, while others also prevent you from getting infected. The latter induce what is known as “Sterilizing immunity”.
With sterilizing immunity, the virus cannot even enter the body because the immune system prevents it from penetrating cells and replicating.
There is a subtle but important difference between to prevent the illness and prevent the infectionon.
A vaccine that “only” prevents the disease may not prevent you from passing it on to others, even if you feel fine. But a vaccine that provides sterilizing immunity stops the virus in its tracks.
In an ideal world, all vaccines would induce sterile immunity. But in reality, it is extremely difficult to produce vaccines that completely stop infection by a virus.
Most of the vaccines that are routinely used today they do not achieve this.
For example, vaccines against rotavirus, a common cause of diarrhea in babies, can only prevent the disease from developing seriously. But still, this has proven invaluable in controlling the virus.
In the United States, there have been nearly 90% fewer cases of rotavirus-associated hospital visits since the vaccine was introduced in 2006.
A similar situation occurs with current poliovirus vaccines, but there is hope that this virus can be eradicated globally.
The first licensed SARS-CoV-2 vaccines have been shown to be highly effective in reducing the disease.
Despite this, we still do not know if these vaccines can induce sterilizing immunity.
Data on this unknown (which will come from ongoing vaccine clinical trials) is expected to be available soon.
Although even if sterilizing immunity is initially induced, this can change over time as the immune response decreases and viral evolution occurs.
Individual level immunity
What would the lack of sterilizing immunity mean for those vaccinated with the new covid vaccines?
Simply put, it means that if you come across the virus after being vaccinated, it can infect you, but you may not have symptoms.
This is because the immune response induced by the vaccine cannot stop the replication of each of the viral particles.
You need a particular type of antibody known as “Neutralizing antibody” to generate sterilizing immunity.
These antibodies block the entry of the virus into cells and prevent any replication. However, the infecting virus might have to be identical to the vaccinia virus to induce the perfect antibody.
Fortunately, our immune response to vaccines involves many different cells and components of the immune system.
Even if the antibody response is not optimal, other aspects of immune memory can be activated when the virus invades the body.
These include cytotoxic T cells and non-neutralizing antibodies. The viral replication will slow down and consequently the disease will be reduced.
We know this from years of study on flu vaccines. These vaccines typically induce protection against disease, but not necessarily protection against infection.
This is largely due to different strains circulating influenza, a situation that can also occur with SARS-CoV-2.
It is reassuring to note that influenza vaccines, despite failing to induce sterilizing immunity, are still extremely valuable in controlling the virus.
Immunity in the population
In the absence of sterilizing immunity, what effect could SARS-CoV-2 vaccines have on the spread of a virus through a population?
If asymptomatic infections are possible after vaccination, there is concern that SARS-CoV-2 will simply continue to infect as many people as before. Is this possible?
Asymptomatic infected persons usually produce virus at lower levels.
Although there is no perfect relationship, more virus generally equates to more disease.
Therefore, vaccinated people have less likely to transmit enough virus enough to cause serious illness.
This in turn means that people infected in this situation will transmit less virus to the next person.
This has been clearly demonstrated experimentally using a vaccine against a different virus in chickens; when only part of a flock was vaccinated, the unvaccinated birds suffered milder disease and produced less virus.
Therefore, although sterilizing immunity is often the ultimate goal of vaccine design, it is rarely achieved.
Fortunately, this has not stopped many different vaccines from substantially reducing the number of cases of virus infections in the past.
By reducing disease levels in people, you also spread is reduced of the virus across populations, and this will hopefully bring the current pandemic under control.
*This artThe article was published on The Conversation and reproduced here under the Creative Commons license. Click here to read the original version.
Sarah L Caddy is inresearcher clunique in viral immunology and veterinary surgeon at the University of Cambridge, UK.
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