The Moderna and Pfizer mRNA vaccines need two shots to achieve the 95% effectiveness against COVID-19 noted in their clinical trials. However, the plodding pace of vaccine distribution due to product shortages and fragmented healthcare systems might delay the timing of the second shot.

The Pfizer drug requires its booster shot to be given 21 days after the first jab, and the Moderna vaccine's second dose is recommended 28 days after from the first.

According to Quartz, postponing the second shot might not pose a huge problem, said immunologist Dr. David Topham, a professor in the department of Microbiology and Immunology at the University of Rochester in New York. But it is important to get the second dose of the vaccine within six weeks of the initial vaccination. While the first dose offers some protection, it is the second dose that can prevent more than 9 out of 10 cases of COVID-19, according to clinical trials.

The first dose is like a "meet and greet" to the immune system, transferring data on how to thwart the virus that causes COVID-19. The second dose fine tunes the immune response to promote even greater and potentially more durable protection, according to Quartz.

The U.S. Food and Drug Administration issued a press announcement saying changes to the dosing schedule "is premature and not rooted solidly in the available evidence. Without appropriate data supporting such changes in vaccine administration, we run a significant risk of placing public health at risk."

However, the Centers for Disease Control and Prevention (CDC) allows for a little more flexibility in administering the second vaccine dose. The CDC says the follow-up dose could be given within 4 days earlier than then scheduled date, and up to 42 days, or 6 weeks, after the first shot. However, the agency noted, if the second dose of these mRNA vaccines by Pfizer and Moderna are "administered beyond these intervals, there is no need to restart the series," adding the staying within the expanded window allows a "permissible risk."

The CDC emphasized, what is important is both doses come from the same manufacturer.