Vaccinated people half as likely to be infected with COVID-19, new study says

LONDON - Fully vaccinated people are half as likely to be infected with the novel coronavirus than those who have not been vaccinated, according to a new study published Wednesday. 

Researchers at Imperial College London analyzed data from more than 98,000 swab tests taken between June 24 and July 12. 

The 37-page study showed participants who reported being vaccinated were at substantially reduced risk of testing positive for COVID-19 compared with those who reported not being vaccinated. 

According to the study, swab positivity for COVID-19 among unvaccinated individuals was three times greater for all ages compared to those reporting two doses of the vaccine.

"Our estimate of vaccine effectiveness against all SARS-CoV-2 infections for two doses of vaccine was 49% in the most recent data, increasing to 58% when we defined effectiveness only for strong positives," the study authors wrote.

Researchers also determined that the chance of a vaccinated person in contact with an infected person testing positive for COVID-19 was 1 in 26. For an unvaccinated person, the chance of becoming infected increased substantially to 1 in 13.

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Meanwhile, those with two shots of the vaccine also seemed to have smaller amounts of virus in the data samples, suggesting they are less likely to infect others they come into contact with.

Scientists found the highest prevalence of infection was among 12 to 24-year-olds, raising the prospect that vaccinating more of the age group could substantially reduce transmission potential in the fall season when schools reopen. 

"We show that the third wave of infections in England was being driven primarily by the Delta variant in younger, unvaccinated people," the study authors continued.

The new study comes amid a new surge in COVID-19 infections associated with the alpha and delta variants. 

Projections released last month by the COVID-19 Scenario Modeling Hub showed a continued and accelerated increase in COVID-19 cases and deaths, with numbers peaking in mid-October to around 60,000 cases and around 850 deaths per day in the most likely scenario.

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Both Monday and Tuesday, the United States recorded more than 100,000 new daily COVID-19 cases, according to data collected by Johns Hopkins University. On Aug. 3, the U.S. recorded 127,976 new daily COVID-19 cases and on Aug. 4, the country counted 106,557 new cases.

Researchers noted that the development of a new vaccine specifically targeting the delta variant may be needed in light of the study’s findings, but existing vaccines are still believed by experts to offer substantial protection against most coronavirus variants currently circulating.

According to a separate study published in the New England Journal of Medicine, researchers found with the Pfizer vaccine, the effectiveness of two doses was 93.7% among people with the alpha variant and 88% among those with the delta variant.

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"Overall, we found high levels of vaccine effectiveness against symptomatic disease with the delta variant after the receipt of two doses," the study authors wrote. 

A study from Public Health England in May also found the Pfizer vaccine 88% effective against symptomatic disease from the delta variant two weeks after the second dose.

Data released by Israel’s Health Ministry in early July said the Pfizer vaccine appeared to be 64% effective against preventing symptoms of the fast-spreading delta variant, but still showed the vaccine is highly effective against preventing serious symptoms and hospitalization. During May, that figure stood at 98.2%, and during June, it was 93%.