Pfizer and BioNTech’s Covid-19 vaccine appears to largely prevent severe disease from the coronavirus omicron variants sublineages BA.4 and BA.5, a study by South Africa’s biggest health insurer shows.

The two-shot Pfizer course offers 87% protection against being hospitalized with the strains that are driving the country’s fifth wave of infections, Discovery Health, basing its study on the more than 1-million clients it has in Gauteng province. That level of protection was based on infection in patients one to two months after receiving their second dose.

The study showed that protection waned to 84% three to four months after the second dose and 63% at five to six months post inoculation, Ryan Noach, the company’s chief executive officer, said in a LinkedIn post dated June 6. A third dose, or booster, improved protection to 85% three months after the dose, with that climbing to 88% by four months, Noach said, citing research led by Shirley Collie, Discovery’s chief health analytics actuary.

“Vaccine effectiveness against severe disease is maintained,” Collie said in an interview.

South Africa has served as a harbinger of how the coronavirus omicron variant and its sublineages may play out in other countries. Omicron was discovered by Southern African scientists in November and South Africa was the first country to experience a wave of infections driven by the variant. South African scientists discovered BA.4 and BA.5 in April and those are now the dominant cause of infection in the country.

South Africa began offering a fourth dose of Pfizer’s shot to people over 50 this week.

Johannesburg’s Business Day reported on the study first. The research is yet to be peer reviewed.

South Africa’s population also has a high level of protection from prior infections. A blood donor survey released last month showed that even before the fifth wave 87% of people in the country had been infected with Covid-19.