In the global race to contain the coronavirus pandemic, there is hopeful news on the vaccine front, with a number of potential candidates being developed and some promising early results. Based on what we know so far, it currently seems likely that most potential vaccines designed to protect against the SARS-CoV-2 virus that causes Covid-19 will require boosters, perhaps regularly. Why is this?
When an infectious agent enters the body, the immune system will notice this and create a memory, so that the next time it encounters the agent there will be a swift, repelling response. In the case of most infectious agents, such as viruses, natural infection produces a long-lasting memory. But this is not always the case.
Two types of vaccine
Some vaccines are made by disabling the infectious agent in some way so that it becomes safe to introduce to our bodies, but still goes through its normal life cycle. The theory is that this will stimulate something close to the natural immune response and produce the long-lasting memory without making the recipient sick.
This is the basis of the vaccine we are given for measles, mumps and rubella – or MMR. It contains live but disabled versions of each virus. Children are given two doses of the vaccine a few years apart. This is in case the vaccine does not “take” the first time around and the immune system needs a reminder of what the viruses look like. This repeat vaccine is not technically a booster, but rather a second dose which allows for possible interference by other childhood infections the first time around, and because a pre-school child’s immune system is still developing.
The MMR approach has been possible because the viruses that cause measles, mumps and rubella are well established in the human population and virologists know a lot about how they interact with the human immune system. But it takes years to create a safe and effective live vaccine, so for SARS-CoV-2, research teams are trying different routes. A good approach is to use a killed version of the virus rather than a modified, live version as in the case of MMR.
The inactivated polio vaccine and influenza vaccines both use killed viruses. The drawback of these vaccines is that the immune response does not last, which is why boosters are needed.
In the case of this vaccine, regular – perhaps annual – boosters would probably be needed to help to ensure people keep their immune memory. In an outbreak situation, everyone in the affected area could be given a dose of the vaccine to help contain the transmission.The Covid-19 vaccine developed by the team at Oxford University, which has shown promising early results uses a broadly similar approach, in that researchers have taken the code for the SARS-CoV-2 “spike protein” and put it into a harmless virus carrier.
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