Mass. will soon have over 4 million residents who’ve gotten at least one dose of the vaccine. So what does that mean?

People photographed the lilacs at the Arnold Arboretum on Friday.John Tlumacki/Globe Staff

Massachusetts will soon cross the threshold of having more than 4 million residents with at least one dose of the COVID-19 vaccine — meaning almost 58 percent the population has some measure of immunity to the deadly virus. But just how important is that benchmark?

Let’s take a peek at the data to find out.

When the vaccine rollout began, Governor Charlie Baker set a goal of fully vaccinating 4.1 million Massachusetts residents out of a total population of about 6.9 million people. The thinking was that getting that many people vaccinated was a “reasonable, herd immunity-approaching strategy for the adult eligible population,” Dr. Paul Biddinger, chairman of the state’s COVID-19 vaccine advisory group, told the Globe in February.

In terms of shots, Massachusetts hasn’t hit that milestone yet. But as of Friday, the state was reporting a tally of 3.7 million first doses of Pfizer or Moderna administered, plus 218,831 total Johnson & Johnson shots. That’s 3,977,531 residents who are now either partially or completely vaccinated — just shy of 4 million.

And hitting that benchmark is noteworthy, according to Northeastern epidemiologist Samuel V. Scarpino.

Scarpino said in an e-mail that vaccinating 4 million residents, even partially, “in only a few months is an incredible accomplishment. Achieving this level of coverage is also how we prevented a larger B.1.1.7-related [variant] surge and how we saved hundreds of lives. However, we need to remain focused on ensuring that we get to 80 or 90 percent of the entire state population fully vaccinated by late September when respiratory disease season restarts.”

Scarpino noted, however, that the fate of Massachusetts is ultimately linked to the fortunes of the rest of the globe.

“We also know there are still serious issues associated with equity, which we need to work diligently to address,” Scarpino wrote. “Perhaps even more importantly, many countries, e.g., India, Nepal, Thailand, are experiencing devastating outbreaks of COVID-19. We need to provide resource to those countries, to include vaccines, urgently.”

Dr. Abraar Karan, an internal medicine physician at Brigham and Women’s Hospital and Harvard Medical School, shared Scarpino’s enthusiasm about the pace of vaccinations in Massachusetts.

“I think it is indeed a big deal — we have close to 40% fully vaccinated here; and 60% with at least one shot; we are in the top tier in terms of coverage among US states,” Karan wrote in an e-mail.

Residents are considered fully vaccinated once they receive two doses of either the Pfizer or Moderna injection, spread out over a period of weeks, or the single-shot Johnson & Johnson vaccine. As of Friday, Massachusetts had more than 2.8 million fully vaccinated residents, out of about 6.5 million doses administered, according to the official state breakdown.

Public health experts and state officials have touted the ongoing vaccination drive as a key strategy in helping Massachusetts residents slowly return to a pre-pandemic lifestyle.

“Some places in the US have high degree of immunity and likely will hit herd immunity with more vaccinations,” tweeted Dr. Ashish K. Jha, dean of the Brown University School of Public Health, in a thread on Friday morning.

Massachusetts, Baker told reporters during a briefing Thursday, is outpacing the rest of the nation on vaccines, which could forecast a summer with comparatively fewer restrictions than last year.

“We continue to lead the nation with respect to our vaccination effort, per capita,” Baker said.

So should we expect to belly-flop on that slip-and-slide in July with large, maskless crowds cheering us on in the park between sips of Sam Summer? That depends.

“Based on the public health data, which continues to move in a positive direction, our goal is to have all limits and restrictions wind down by the middle of the summer,” Baker told reporters.

Karan said loosening restrictions is reasonable as vaccination numbers go up.

“I think that restrictions should absolutely be correlated with the percent of folks who are vaccinated along with other measures like cases per 100,000 and test positivity as well,” Karan wrote. “As of now, Massachusetts is set to loosen more restrictions on May 10 and 29th; and then August 1st thereafter, but ... the August 1st date is not fixed and could be re-evaluated pending vaccinations and data on new infections. Also, as the weather improves, I expect that more outdoor activities will also help to further drive down transmission.”

A word of caution for the vaccinated, though: People have different levels of immunity to the virus based on whether they’re fully or only partially inoculated.

The CDC says that data show “receiving only one dose” of Pfizer or Moderna “provides some protection against COVID-19, at least in the short term.”

How much?

Well, the CDC said in a March Morbidity and Mortality Weekly Report that an analysis of two skilled nursing facilities in Connecticut showed one dose of Pfizer was 63 percent effective against infection.

To receive the most protection, Americans “should receive both recommended doses,” the CDC says.

If they get both, the protection increases substantially, with the CDC reporting in late March that the two-shot Pfizer or Moderna regimen provides as much as 90 percent protection against infection.

That’s good news for Massachusetts residents, some 99 percent of whom have gone back to get their second dose, according to a statement released in late April from the state’s COVID-19 Command Center.

And even if someone catches COVID while fully vaccinated, the shots will likely shield them from the worst possible outcomes, according to the CDC.

“Some people who are fully vaccinated against COVID-19 will still get sick because the vaccines are not 100% effective,” the agency says. “When this happens, vaccination might help keep you from getting seriously ill, based on data from clinical studies.”

There’s also the thorny question of emerging variants of the virus, and how effective the vaccines approved in the US will ultimately be against them.

So far, so good — with caveats, according to the CDC.

“New variants of the virus that causes COVID-19 are spreading in the United States,” the agency says. “Current data suggest that COVID-19 vaccines authorized for use in the United States offer protection against most variants. However, some variants might cause illness in some people after they are fully vaccinated.”

Ultimately, the slow progress made on the vaccine front has some public health specialists bullish on what the landscape could begin to look like, despite the ever-shifting goal posts around herd immunity.

“I know many people were panicked when [they] read that we may not reach herd immunity,” tweeted Dr. Jennifer Nuzzo, an epidemiologist at the Johns Hopkins Center for Health Security, on Friday morning. “But trust that we will return to normal.”

Material from Bloomberg and prior Globe stories was used in this report.