Till date India  presently approved three Covid-19 vaccines for the use in the country ie.,  Serum Institute of India’s Covishield, Bharat Biotech’s Covaxin and Russia’s Sputnik V, which are all two-dose vaccines.
As per the data ascertained by  Union  government of india  has recommended a gap of 12-16 weeks for Covishield, while it is 4-6 weeks for Covaxin and 21-90 days for Sputnik V. A person is fully vaccinated after receiving the first dose, but the bigger question remains,  Let is see from the point of view of experts .. 
With respect to question of how Long it Takes to Build Immunity after Jab..
World Health Organisation’s Dr Katherine O’Brien says that after the first dose, a good immune response kicks in about two weeks. But the immune response boost boosts after the second dose, making a person’s immunity even stronger.
With respect to question of how Long Does the Immunity Last..
Dr Katherine says we don’t know yet how long the immunity lasts after taking the vaccines and adds that it is going to take some time to know the details.
“We are following people who have received vaccinations to find out whether their immune response is durable over time and the length of time for which they are protected against the disease. So we are really going to have to wait for time to pass to see just how long these vaccines last,” the WHO scientist said.
However, what is known is that the two-dose of Pfizer vaccine remains effective for at least six months or even longer.
Similarly, Moderna vaccine were also found to have antibodies six months after the second shot.
With the Covishield vaccine being administered in India, the immune responses that can persist for a year or more.
“At this point, we cannot say, but other vaccines using the Oxford ChAdOx1 technology are proven to provide immune responses that can persist for a year or more,” she says.
Bharat Biotech, the company that manufactures Covaxin in India, has already started trials for booster dose.
The booster dose is to be given six months after the participants had received their second dose in the trial, which had taken place between September and October 2020.
However, a report in The New York Times quoted two recent studies that have found that immunity to the coronavirus lasts at least a year, possibly a lifetime after vaccination.
This means that person recovering from Covid-19 and later fully immunised will not need a booster shot at all.
With respect to question of possibilty of New Variant Reduce Immunity ..
As the new variants have been detected across the globe and in India as well, there have been reports that the vaccine efficacy against the variants including B.1.617.1 and B.1.617.2 first detected in India.
Scientists say that a third booster shot is required after the two dose.
It is important to understand that fully vaccinated against Covid-19 does not guarantee protection against Covid-19 again.
Since the Coronavirus is new and mutating, it is unpredictable.
AIIMS Director Dr Randeep Guleria said, “This virus is very clever and keeps mutating, we cannot say what will be the protection from vaccines as far as new emerging variants are concerned.” He added that masks and distancing work against all variants.