Why you really need the second dose of a vaccine!

Not surprisingly there has been a big push to do away with the second dose of the COVID-19 vaccines. The concept behind this is to immunize twice as many people with a single dose of the vaccine. The United Kingdom has decided to take this approach and delay all second doses of COVID-19 vaccine potentially for three months! Others simply want to avoid getting stuck twice.

Fortunately, in the United States the FDA has remained strongly opposed to this idea. President-elect Biden has vowed to release all available vaccine for initial doses, but importantly not to do away with second doses.

The problem is most people don’t understand that the second shot of a vaccine is not simply insurance of immunity, but is essential to the development of effective and particularly long-lived immunity.  Delaying a second dose not only jeopardizes effective immunity, but may even result in tolerating and preventing the induction of effective immunity in the future. This would put people at even greater risk than no vaccine at all!

Let me explain.

The first exposure to a virus (or a vaccine) wakes up your immune system. It allows your body to recognize the specific proteins produced by the virus and to start making an immune response. This includes blood proteins (antibodies) that neutralize virus and importantly lymphocytes that kill virus infected cells.  Both components are necessary to affectively defeat the virus and allow a human to survive the infection. 

Unfortunately, this primary response is effective only as early protection against infection. If there is no ongoing exposure to the virus or vaccine, the immunity wanes and is actually turned off to prevent problems like your immune system attacking yourself (autoimmune disease). 

This means that the immunity from a single vaccine dose is not long lived and may not be summoned again if there is a second viral infection. You can actually make an infection worse with a vaccine if the induced immunity is not strong and effective (called vaccine enhancement).

The second shot of a vaccine is crucial both in terms of its timing and its dose. You need the second exposure within the period of time when your immune system is still activated from the first exposure. This is so all of the lymphocytes that responded initially are still there waiting for a second kick start. This is normally three to four weeks after the first dose, unless you’re using some type of vaccine like a live virus that persist over a long period of time. 

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Therefore, the decision to give a second dose of either the Pfizer or Moderna vaccines three to four weeks after the initial dose was not some random concept, but based specifically on well-known immune theory that has evolved over hundreds of years.

Some have suggested that there was evidence of immunity after a single dose of these vaccines, but that immunity was much less effective and short lived. It is bothersome to me as an immunologist that individuals in the U.K. with scientific backgrounds who should know better are pushing this idea.

Get the vaccine and make sure you get the second dose on time! It is the only assurance of long-term protection. You will need it!