The percentage of people who failed to get the second dose of Pfizer or Moderna COVID-19 vaccines is growing, according to recent CDC data. Some five million people, or almost 8% of people who got the first vaccine dose, missed getting the second.

In mid-March, 3% of people were overdue for their second dose.

People who missed getting their second vaccine dose provided several explanations. Some wanted to avoid adverse events, which are more common after the second dose. Others believed a single dose offered sufficient protection.

Another contributing factor relates to supply constraints at vaccination sites, which are instructed not to use Moderna and Pfizer vaccines interchangeably.

The supply constraints are likely to ease the number of people making appointments for COVID-19 vaccination falls in many parts of the country.

Some states, including Arkansas and Illinois, are using phone calls, text messages and letters to persuade the partially vaccinated to get the second dose. South Carolina is amassing a supply of thousands of doses for those overdue for their second dose.

In states such as Massachusetts, compliance was higher. Some 99% of vaccine recipients had received their second dose within 42 days of receiving the first.

CDC guidelines call for people to receive the second dose within 42 days, or six weeks. Moderna recommends a 28-day gap between vaccine doses while Pfizer specifies 21 days.

Researchers don’t have a solid grasp of the degree of immunity a single vaccine dose offers. A CDC study found that people who had a single dose of vaccines from either Pfizer or Moderna had 80% protection. The duration of that immunity, however, is unclear.