Pfizer's top scientist has said that the company plans to request U.S. regulators for authorization to administer a booster dose of its Covid-19 vaccine as soon as August.

The announcement was made in response to evidence of a higher risk of reinfection six months following inoculation, as well as the emergence of the extremely infectious Delta variant of coronavirus.

According to Pfizer's chief scientific officer, Mikael Dolsten, the U.S. pharmaceutical company and its German partner BioNTech have begun creating a version of their vaccine particularly to tackle the extremely contagious Delta variant.

However, in an unusual move, two key government authorities said that Americans do not require boosters at this time and that it is not up to companies to determine when they may be required.

Several hours after Pfizer issued its statement, the Food and Drug Administration and the Centers for Disease Control and Prevention issued a joint statement stating that Americans do not require booster injections at this time.

"Americans who have been fully vaccinated do not need a booster shot at this time," they said.

According to Pfizer and its partner BioNTech, evidence is mounting showing people's immunity begins to diminish after being vaccinated. The Pfizer vaccine requires two doses to provide full immunity.

"As seen in real world data released from the Israel Ministry of Health, vaccine efficacy in preventing both infection and symptomatic disease has declined six months post-vaccination, although efficacy in preventing serious illnesses remains high," Pfizer said in a statement emailed to CNN.

U.S. government officials have underlined that fully vaccinated persons are at minimal risk of infection, even from the Delta variant, which is more transmissible than earlier lineages of the virus.

Furthermore, several studies have found that Pfizer and Moderna's mRNA vaccines provide long-term protection.

Pfizer CEO Albert Bourla said in April that people will most likely require a booster dose of the company's vaccine every 12 months, similar to an annual influenza vaccination. However, some scientists wonder when or if such shots will be needed.