Four reasons why the leading COVID-19 vaccines may not be the best vaccines in the long-run.

1. Moderna’s RNA vaccine has to be kept at –20 °C. Pfizer RNA vaccine has to be kept at–70 °C.

This is uncommon for vaccines, because these are not like other vaccines in common use.

2. A successful vaccine may be 50% effective in reducing severity, and not in stopping infection altogether. That's the minimum criterion for success that has been set by the U.S. F.D.A.

3. None of the leading vaccines have been tested in small children. The hallmark of a successful vaccination program is vaccination of small children. The safety profile of these vaccines in pregnant women is unclear as well.

4. Only one of the COVID-19 vaccines uses one-shot. All of the others require two shots. A two-shot vaccination for an acute disease without the promise of lifelong immunity might not seem attractive to many people.

Other issues (apart from the cold-chain needed) include the need for vials, adjuvants, and syringes for billions of doses. Manufacturers are gearing up for this, but almost certainly there will be snags.

The anticipated date of the Emergency Use Authorization for the first vaccines has already been pushed back to December. All of it depends on the interim data for the phase III clinical trials underway right now.

If all goes well, broader distribution may start in January 2021.

Putting this together, it is unlikely that most countries will reach 70% threshold of vaccination in 2021.