pfizer‘sand Art NouveauOmicron boosters reduced the risk of mild disease from sub-variants of the XBB family compared to people who did not receive the vaccine, according to a study by the Centers for Disease Control and Prevention released Wednesday.
The CDC study provides the first estimate of the real effectiveness of omicron shots against a family of XBB sub-variants. Some scientists warn that XBB subvariants could trigger a new wave of Covid because they are so good at evading antibodies that block infections.
In people aged 18 to 49 years, the omicron booster reduced the risk of mild illness with XBB subvariants by about 48% two to three months after vaccination. According to the study, vaccinations provided 38% protection against mild illness for people aged 50 to 64 and 42% for people aged 65 and over.
CDC officials, speaking to reporters on Wednesday, said the results of the study were encouraging because people who received boosters had more protection than those who didn’t. According to them, protection against serious diseases should be higher, although such data are not yet available.
“This reduces the risk of symptomatic infection by about half at the population level,” Dr. Ruth Link-Gelles, a CDC official and author of the study, said of the 18 to 49 age group.
“We know from past experience that vaccines generally provide better protection against more severe diseases,” Link-Gelles said. “So these are estimates for symptomatic infection, and we expect similar hospitalization and death estimates to be higher.”
Sub-variant XBB.1.5 is rapidly becoming dominant in the US and currently accounts for about 49% of new Covid cases nationwide. World Health Organization officials have described XBB.1.5 as the most contagious version of the virus, although it does not have mutations that would indicate that it makes people sicker than other sub-variants.
Scientists have found that XBB.1.5 is highly immune elusive and has mutations that allow it to better bind to human cells. But CDC research shows that omicron boosters provide the same protection against the XBB family as they do against BA.5 and its descendants such as BQ.1 and BQ.1.1.
For example, in adults aged 18 to 49 years who received a booster dose of omicron, the risk of mild disease from BA.5 and its offspring was reduced by 52%, compared with a 48% risk reduction for the XBB family.
“We did not observe a reduction in vaccine protection against symptomatic disease for XBB and XBB.1.5 compared to other recent BA.5 options,” said Dr. Brendan Jackson, head of CDC’s Covid-19 response.
The study compared people who received the new booster vaccine with those who received two to four doses of the original vaccine. The boosters target omicron BA.5 and the original strain of Covid that emerged in Wuhan, China, while the older shots only target the original strain of the virus.
People who only received original shots typically received their last dose about 13 months ago. They had very little protection against mild illness due to the weakened immunity seen with older vaccines, Link-Gelles said. It is still too early to draw conclusions about how long protection from Omicron boosters lasts, she said.
“Even though your protection against a symptomatic infection may decrease over time, you are likely still protected from a more serious illness for a longer period of time,” Link-Gelles said.
The CDC study looked at the Covid test results of approximately 29,000 people from December to January 13. During this period, XBB.1.5 rose from 2.4% of cases to about 37%. Among more than 13,000 people who tested positive, 78% became infected with the BA.5-associated subvariant and 22% became infected with the XBB-associated subvariant.
The study did not conduct a detailed genomic analysis of each positive test sample to determine with 100% certainty which subvariant caused the infection. Instead, the scientists used the quark in PCR testing to determine which variant likely caused the infection.
The BA.5-related subvariants have a mutation that deletes a gene on the virus spike targeted by the tests, while the XBB subvariants do not have this deletion. If the gene is found, it is probably XBB, and if it is not found, then it is probably related to BA.5.