The US Centers for Disease Control and Prevention released data on Sunday which shows millions of Americans are skipping their second shot of the COVID vaccine.

The report published by the CDC shows roughly 5 million Americans who had one shot of the COVID vaccine from Pfizer/BioNTech or Moderna skipped the second dose. The number is roughly 8% of all the people who had been vaccinated about the completion stage of the survey through April 9.

The New York Times reported that some people who missed the second shot said they missed it because they thought the first shot protected them enough. While the others said they were afraid of the possible side effects, which include pain and swelling at the injection site, fatigue, muscle pains, fever, headache, and nausea. 

The Moderna and Pfizer vaccines need to be administered in two doses to be fully effective. One shot of either vaccine would give patients only 60% protection from COVID-19. A second shot boosts protection to over 94%. 

Evidence collected in vaccine trials and real-world immunization campaigns suggests that a single shot may leave patients more vulnerable to dangerous COVID-19 variants. It is also unclear how long protection against the coronavirus will last for people who only receive the first dose. 

The CDC noted that patients who haven’t received their second jab during the three- or four-week window can still get it. And patients would still be considered fully vaccinated even if they received the second dose six weeks after the initial shot.