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On Two Dose Vaccine

In the United States, many people have received either Pfizer or Moderna COVID-19 vaccines. These vaccines require two doses spaced one month apart. There is a period between the first dose, when the vaccine provides some degree of protection, to after the second dose when the vaccine protection is maximized. In public, this is often described as partially vaccinated v.s. fully vaccinated. The label partially vaccinated has a connotation of temporary and insufficient. I believe this is not accurate. I suggest a different set of labels, vaccinated and double vaccinated, to describe these two stages. This label could help shape public perception and health policy and improve outcome.

First of all, the maxim of biology is that nothing is ever binary. Everything is really different shades of gray. In this respect, the label fully vaccinated also has an inaccurate connotation. Rather than making you 100% safe from COVID, it should be understood as having stronger protection, not a 0 and 1 difference. Pfizer and Moderna are impressively effective vaccines. Real world data suggest just one dose of vaccine reduce the chance of inflection by 80%. This is higher than the 70% effectiveness claimed by the alternative J&J vaccine. It should not be thought of as insufficient.

How do people decide there should be two doses of vaccine spaced one month apart? Is it better or worse than spacing three months or six months apart? Why is two doses instead of one dose? Could three doses do even better? There is not really a correct answer. The way it is decided can be described as trial and error. Scientists experimented with different dosages and schedules and found one that works best. Given the compressed schedule to obtain regulatory approval, the pharma companies do not even have time to test three month or six month spacing. So one month it is. The recommendation is also a balance of cost and benefit. Let’s say a third dose provides an additional 5% protection. This is unlikely to be recommended because the benefit does not justify the cost and the burden of making three visits to a clinic.

There are different strategies to roll out vaccination besides strictly following the one month spacing. Consider 100 people and 100 vaccine doses available. We can either give every one one shot, or we can have 50 people two doses but left nothing for the other 50 people. In the first case, all 100 people have 80% protection. In the second case, 50 people have 90-95% protection, but the other 50 have none. The first case is much more preferable than the second as far as public health’s concern. UK has adopted the first strategy for its vaccine roll out. People are recommended to wait three months for the second dose in order to vaccinate more people. As a result, the COVID case in the UK has plummeted, a sharp contrast to continental Europe.

This is why I suggest to use the label vaccinated and double vaccinated to avoid the connotation of inadequately vaccinated. For public health tracking purposes, the primary metric to consider is the number of people vaccinated. The number of people who have received two doses is much less important than the first.




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