Maine’s average rate of daily vaccinations increased by 87 percent last week, reaching the highest level in roughly four months.

From Sunday, Sept. 26 through Saturday, Oct. 1, Maine administered 28,430 vaccine doses, an average of 4,061 per day. During the previous week, the total was 15,154, or 2,165 per day, according to the Maine Center for Disease Control and Prevention.

The increase was driven largely by third doses that have been administered to older Mainers and those who are immunocompromised, CDC Director Dr. Nirav Shah said Monday on “Maine Calling,” Maine Public’s call-in radio show.

But the timing also suggests that some of the increase can be attributed to the Oct.1 deadline for most health care workers to be fully vaccinated. An agency spokesman said that correlation hasn’t been confirmed but can’t be ruled out.

“At this time, data show that last week’s update in daily vaccinations resulted, in part, from the administration of booster doses, but we can’t confirm that Maine residents who received first and second doses last week were motivated to do so by the health care worker vaccination rule,” CDC spokesman Robert Long said.

Long added that vaccination clinics report to the state whether doses are first, final, or booster doses, but not the employment status of those who receive doses.

The three biggest daily totals last week occurred on Tuesday, Wednesday and Thursday, the final three days of September. On each day, more than 5,000 shots were given. The last time at least 5,000 shots were administered on a single day was June 11.

Although the deadline for health care workers to be fully vaccinated was Oct. 1, state officials have said they won’t begin enforcing the mandate until Oct. 29. After Gov. Janet Mills announced in August that she would require COVID-19 vaccines for health care workers, some protested and others threatened to leave their jobs at a time when staffing challenges already are great.

However, most major hospital and health care systems said vaccination rates among workers have been high and not many workers are following through on their threats. At MaineHealth, just 60 of its 23,000 employees had resigned because of the mandate. At Northern Light Health, 89 of 12,500 have quit, or less than 1 percent of the total workforce.

One woman who was vaccinated last week at the Cumberland County Fair confirmed that her decision was motivated by the state mandate.

“The reason I am doing this is so I can keep my job,” said Andrea Harmon of Oxford, who works as a daily living skills aide.

Overall, Maine has administered 878,363 final doses of vaccine, which accounts for 65.3 percent of all Maine residents and 74.2 percent of those 12 and older, the age group currently eligible to be inoculated. The U.S. Food and Drug Administration could authorize vaccines for children 5-11 as soon as this month.

In addition, the state has begun giving additional doses of the Pfizer vaccine to older Mainers and individuals who are immunocompromised. As of Monday, 29,462 third doses have been administered. The CDC did not have a day-by-day breakdown of third doses. So far, only the Pfizer vaccine has been authorized for a third dose, although the FDA could soon consider authorization for the Moderna and Johnson & Johnson vaccines.

Shah said that health officials are “just as focused on first doses as third doses,” and that reaching those who remain unvaccinated is the key to getting out of the pandemic.

Further, Shah stressed that although vaccinated people can still contract COVID-19 and transmit it, “that pales in comparison to the rate that unvaccinated people can get and transmit COVID.”

Vaccinations also have been increasing across the country, according to a tracker by Bloomberg News, although it’s not clear whether that increase can be attributed to third doses.

As of Sunday, the U.S. was averaging over 832,000 shots per day. That’s a 19 percent increase from about 700,000 shots per day one week ago. Maine trails only Vermont and Connecticut among states with the highest vaccination rate and is more than 12 percentage points above the national rate.

Maine, however, has a big disparity in rates between counties. Cumberland County, for instance, has a vaccination rate of 75.6 percent, followed by Knox and Lincoln counties, which also are above 70 percent. On the other end, eight of Maine’s 16 counties have yet to reach a vaccination rate of 60 percent. Somerset County is the lowest, at 53.5 percent.