Pfizer announced on Thursday that it’s working on developing a booster for its groundbreaking two-dose coronavirus vaccine as it sees effectiveness levels drop over time in those inoculated, CNN reports.

“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,” the company said in a statement emailed to CNN.  “Additionally, during this period the Delta variant is becoming the dominant variant in Israel as well as many other countries. These findings are consistent with an ongoing analysis from the Companies’ Phase 3 study.”

The company said it would seek FDA approval through an emergency use authorization for the booster, which it believes will be introduced in August.

The Pfizer vaccine had an effective rate at 90% when it was introduced last December.  A booster increases the effectiveness of the first doses by five to ten times, and it will help those vaccinated fight off the B.1.617.2 or delta variant.  The original vaccine had a 64% effectiveness rate against the delta variant, according to data from the Israel Health Ministry.

“While Pfizer and BioNTech believe a third dose of BNT162b2 has the potential to preserve the highest levels of protective efficacy against all currently known variants including Delta, the companies are remaining vigilant and are developing an updated version of the Pfizer-BioNTech COVID-19 vaccine that targets the full spike protein of the Delta variant,” the company said.

Most virologists and public health professionals expected that a booster shot would be necessary, even before the delta variant was detected.  Many foresaw an annual vaccination, similar to flu shots, as a necessity given how the coronavirus mutates.