Namely, trying to combine vaccines by giving a 2nd dose of a different vaccine to the one that was first administered.
The reason they are using this newish strategy – called heterologous prime-boost vaccination, in case you want to impress your friends – is that in the face of emerging virus variants of concern, they are hoping this might boost a more effective immune response against the virus that causes Covid-19 than using two shots of a single vaccine.
Plus, if you could effectively combine vaccines, that would ease the supply problem in the case of those vaccines that require a 2nd shot.
AstraZeneca seems to be the leader in this effort after announcing trials that will combine its vaccine with the Russian Sputnik V and with the Pfizer vaccine.
In fact, Sputnik already does that to an extent using different viral carriers in each of its doses.
Pfizer says that it might have preliminary results as early as June so stay tuned.