Delayed second Pfizer-BioNTech shot produces more antibodies, U.K. study says

The Pfizer-BioNTech COVID-19 vaccine generated antibody responses three-and-a-half times larger in older people when a second dose was delayed to 12 weeks after the first, a British study says.

The study, released on Friday, is the first to directly compare immune responses of the Pfizer-BioNTech shot from the three-week dosing interval tested in clinical trials and the extended 12-week interval that British officials have recommended in order to give more vulnerable people some protection quickly.

After Britain moved to extend the interval between doses, Pfizer and vaccine partner BioNTech said there was no data to back up the move. However, Pfizer has said that public health considerations outside of the clinical trials might be taken into consideration.

Canada made a similar move earlier this year, when the National Advisory Committee on Immunization (NACI) recommended extending the duration between first and second COVID-19 vaccination doses to up to 16 weeks in order to give as many people as possible some degree of protection as quickly as possible. 

Like Canada, Britain began rolling out the Pfizer-BioNTech vaccine before changing its dosing policy, meaning a small number of people who got the shot early received the second shot three weeks later, providing a comparison group for the British researchers.  

"Our study demonstrates that peak antibody responses after the second Pfizer vaccine are markedly enhanced in older people when this is delayed to 12 weeks," said Helen Parry, an author of the study and a clinical lecturer with the Institute of Immunology and Immunotherapy at the University of Birmingham. 

The study, which has not yet been peer-reviewed, looked at 175 people aged between 80 and 99, and found that extending the second dose interval to 12 weeks increased the peak antibody response 3.5-fold compared to those who had it at three weeks.

The authors warned against drawing conclusions on how protected individuals were based on which dosing schedule they received. However, taken with data showing good protection against hospitalization and death from just one shot of Pfizer vaccine, Public Health England said the study was further supportive evidence in favour of the delayed-dose approach.

"The approach taken in the U.K. for delaying that second dose has really paid off," Gayatri Amirthalingam, consultant epidemiologist at Public Health England, told reporters.

'Not a surprise,' Canadian vaccine expert says

Experts in Canada said the British findings show that a fundamental principle of how vaccines traditionally work appears to apply in mRNA vaccines like Pfizer-BioNTech. 

"The findings are not a surprise," said Dr. Danuta Skowronski of the BC Centre for Disease Control, whose research helped guide Canada's decision to extend the interval between COVID-19 vaccine shots, in an email to CBC News. 

"We underscored that extending the interval between first and second doses would enable more people to benefit from the substantial protection offered by a single dose without negatively affecting the second dose boost response," Skowronski said. 

"In fact, we anticipated that the boost response would be improved by that extended interval."

Prof. Alyson Kelvin, a virologist at Dalhousie University and the IWK Health Centre in Halifax, agreed that the British study results support Canada's dose-spacing strategy. 

"I feel strongly that the Canadian strategy which prioritized a single shot into as many people as possible has saved an inordinate number of lives," Kelvin said in an email to CBC News.

However, she cautioned that people, particularly older people, must remember that they're not fully protected against COVID-19 during the period between doses.