The MBTA's Vax Express made its way back to Boston as part of an initiative to help increase access to the COVID-19 vaccine and vaccinate those who have yet to receive a dose.The mobile vaccination clinic in an MBTA Commuter Rail train was at South Station on Wednesday after previously stopping there on June 16. The Vax Express, run by CIC Health, also made a stop in Boston's Mattapan neighborhood.CIC Health says about 200 people statewide have received a dose onboard the Vax Express thus far."What we're trying to do at CIC Health is make it as readily accessible as possible by being, literally, in people's backyards," said Samantha Joseph, of CIC Health."It's really important for us to reach communities that have been severely impacted by COVID-19 and to bring the vaccines to them in an environment that may be a little more approachable than going to a hospital or large health care center," said Justin Thompson, a spokesperson for Keolis, which operates the MBTA Commuter Rail.More than 4.2 million people in Massachusetts are fully vaccinated against COVID-19, which amounts to more than 60% of the state's population. But the vaccination rates are still low in communities like Mattapan."This helps because of the fact that we have to bring vaccinations to the people because, clearly, people are not going to the vaccinations," said State Rep. Russell Holmes.Those who get vaccinated on the Vax Express also receive incentives, such as grocery store gift cards and free lunches.The public can walk up without an appointment or book an appointment in advance.The Vax Express offers the two-dose Pfizer vaccine for ages 12 and up and the single-dose Johnson & Johnson vaccine for ages 18 and up.