Answer: There are a lot of reasons why vaccines are preferable to natural immunity (besides not getting sick!). Vaccines have immune enhancing strategies including: delivery of a uniform dose of immune stimulus, two injections and/ or use of an adjuvant (immune enhancer).
You may have seen the recent publication profiled in the NY Times that natural immunity may last longer than previously thought–months or even years. While it is natural for antibodies to decrease in patients post-infection, other types of immune cells (such as T cells and B cells) may still be present to fight the infection should the body encounter SARS-CoV2 again. This is great news for natural infection (although there are a lot of caveats in terms of variability within patients), and also very promising news for the possibility of longer immunity for the vaccine.
The limitation with immunity from natural infection is that each person has a different immune response, and that response is partially based on the level of exposure to the virus. This results in the levels of antibodies and immune cells–broadly, immunity–being variable in each person. While one person may have longer protection, another person may get a mild reinfection but can still pass it on to someone else, and yet another person may have little to no protection against a secondary infection. This is one reason why people who have had COVID-19 do not get an “immunity passport”. However, because vaccines deliver a uniform dose of immune stimulus to each person, there is less variability in the immune response that develops. Vaccines are developed such that the majority of patients develop protective immunity. Already both Pfizer and Moderna vaccines have announced ~95% effectiveness in recent studies.
Secondly, most vaccines in development require two shots. This is an immunology strategy called “prime-boost”. The first injection is used to “prime” the immune system. The immune system recognizes the material in the vaccine as foreign and develops an immune response. Then several weeks later, a second injection is given to “boost” the immune response that was developed, making it stronger. Doing this results in more and longer-lived “memory cells” that can stay around to recognize the virus. The immune system is tricked into arming itself with higher levels of protection.
In the phase 1 studies of the Pfizer vaccine, 7 days after the second dose of the vaccine study participants had much higher levels of circulating antibodies compared to convalescent plasma collected from patients that had natural SARS-CoV-2 infections. This is a demonstration that the vaccine may provide higher protection compared to natural infection with the virus.
All the vaccines in development target the spike protein of SARS-CoV-2. The spike protein is how SARS-CoV-2 grabs onto cells in our body. Researchers believe by focusing on this part of the virus we can make an immune response that is “neutralizing,” which means it stops the virus from entering our cells. By stopping the virus before it enters the cells, it prevents the virus from entering, making copies of itself and spreading in the body. Or, it prevents infection.
Many vaccines in development also include some sort of adjuvant–or as this Nerdy girl describes it–immune tickler. The Janssen vaccine and the Astrazeneca vaccines use inactivated modified common cold viruses that have been altered to express the spike protein of SARS-CoV-2. The cold viruses are there to stimulate the immune system. The GSK/Sanofi and Novavax vaccines are also using spike proteins with adjuvants to stimulate the immune system.
Vaccines have the edge over naturally acquired immunity in terms of creating immune responses that are directed against the part of the virus that can prevent viral entry and through boosting the immune system in multiple ways. We do not know how long immunity will last with the vaccine as these are newer vaccines. Yet scientists are hopeful that the immunity may last at least a year or two, which could get us through the pandemic. Ultimately, it is possible we may need another “booster” vaccine, but it remains to be determined.