Prior COVID-19 infection boosts immune response to variants after one dose of Pfizer vaccine, UK study finds

A new study from the United Kingdom revealed one shot of the Pfizer/BioNTech vaccine boosted protection against COVID-19 variants - but only for those who had been previously infected with the virus.

Researches from Imperial College London, Queen Mary University of London and University College London published their study on Friday.

Scientists looked at the immune responses of health care workers from Barts and Royal Free hospitals after they’d received one shot of the Pfizer/BioNTech vaccine, which requires two doses.

Blood samples were analyzed to measure the immunity against the COVID-19 variants. 

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The data revealed that those participants who had a previous bout with the coronavirus and one shot of the vaccine had "enhanced protection" against the UK and South African COVID-19 variants. The study pointed out that participants had more T-cells, B-cells and antibodies, which aid in preventing an infection. 

Those participants who had one dose of the vaccine but hadn’t previously contracted the coronavirus showed a weaker immune response to the variants, according to the study. The study showed they produced lower levels of antibodies than the previous group even after the first dose of the vaccine, leaving them with an immune response that could be insufficient against the variants. 

"Our findings show that people who have had their first dose of vaccine, and who have not previously been infected with SARS-CoV-2, are not fully protected against the circulating variants of concern," Imperial College London Professor Rosemary Boyton said. "This study highlights the importance of getting second doses of the vaccine rolled out to protect the population."

The Centers for Disease Control and Prevention has always maintained that the two-dose regimen for both Pfizer/BioNTech’s and Moderna’s vaccine is best, even when some lawmakers and health officials urged the U.S. to shift to a single-dose regimen to speed up protection.

However, getting people to get their second shot of the COVID-19 vaccine remains a challenge.

More than five million Americans have missed out on their second COVID-19 vaccine dose from Pfizer/BioNTech and Moderna, according to the U.S. Centers for Disease Control and Prevention.

RELATED: Millions of Americans are missing ‘critical’ second COVID-19 vaccine shot, according to CDC data

According to the CDC, more than 101 million Americans have been fully vaccinated, representing 30.5% of the total U.S. population. The agency said the five million people who haven’t received their second dose within the recommended timeframe represent nearly 8% of those who got their first shot of either Pfizer/BioNTech or Moderna’s vaccine, meaning about 92% of people are completing their vaccine.

But the agency said it’s not surprising that people have skipped or missed out on getting their second dose.

"Overall, this increase in missed second doses was expected as eligibility expands to more people," the agency said in a statement to FOX Television Stations last week. "The groups initially prioritized for COVID-19 vaccination were more likely to have been vaccinated at their work site (healthcare providers) or residence (long-term care facilities), potentially reducing barriers and increasing adherence to the recommended vaccine schedule."

While it was expected, the agency said the exact reason for why people missed their second shot still needs further research.

But "vaccine hesitancy" remains a problem as the overall pace of vaccinations has slowed over the past several days. According to the Wall Street Journal, the country administered an averaged of 3.2 million shots a day in early April. That number has dipped to 2.8 million doses a day, according to the outlet.

CDC Director Rochelle Walensky said the agency is working to educate people about the science, data and safety of vaccines.

"If people are worried about the side effects, we can convey the data of over 200 million vaccine doses and the safety and the scrutiny of that safety," she said. "So we need to meet people where they are and understand why they might be hesitant and then give them the information that combats that hesitancy."

This story was reported from Los Angeles.