My Second Dose of Covid Vaccine Has Been Delayed, is this a Problem?

Maybe, but maybe not. First off, it’s worth acknowledging that getting the first dose of the COVID-19 vaccine is better than getting nothing at all. The most recent studies suggest that, 14 days after receiving the first dose, patients’ risk of contracting the virus will still be 75 percent lower for both the Pfizer and AstraZeneca vaccines, and 80 percent lower for Moderna. 

That is high enough that some organizations and public health professionals believe that, to contain the spread of the virus, it is most important to roll out first doses to a wider percentage of the population. In January the U.K. government decided to prioritize first doses, saying that their models suggest that “initially vaccinating a greater number of people with a single dose will prevent more deaths and hospitalizations than vaccinating a smaller number of people with two doses.”

One aspect that’s still very troubling, though, is that having huge numbers of citizens who are only partially immunized could actually speed up the pace of mutation for more dangerous coronavirus variants. Earlier in the pandemic, COVID-19 wasn’t induced to evolve because none of its carriers’ immune systems had developed antibodies. But now that millions of people have been infected across the globe, mutations are rapidly emerging – and one logical outcome of partial immunizations is a further acceleration in that evolution of mutations. 

That concern is one of several that has spurred many countries to proceed in the process of getting people fully vaccinated, to the tune of 95 percent effectiveness. The downside, of course, is that this has inevitably led to major delays. As of February 26, 2021, a CBS report found that roughly one out of every eight Americans who received the first dose of the Moderna vaccine – a total of nearly three million people – hadn’t gotten their second dose within 28 days, which is what was initially recommended by the U.S. Centers for Disease Control and Prevention.

However, there’s some recent good news from the CDC, which in February updated its guidance to say that people taking the Moderna vaccine can wait up to 42 days between doses.The jury is still out on how effective the second dose is if it’s injected later than that: one encouraging study published by the developers of the AstraZeneca vaccine suggests that their shot may actually perform better when its doses are spaced apart by 84 days, or 12 weeks. (The paper has not yet been peer-reviewed, so its findings should certainly be taken with a grain of salt.)

Jackie Phillips, MD, March 1, 2021