UK report suggests booster effectiveness against symptomatic omicron decreases within 10 weeks

A United Kingdom Health Security Agency report published on Thursday suggests that the effectiveness of the COVID-19 booster against symptomatic infections of the omicron variant wanes within 10 weeks.

The report included 147,597 delta and 68,489 omicron cases, which were reported between Nov. 27 and Dec. 17. It analyzed the percentage of vaccine effectiveness between reported cases of the two strains in individuals who had received a two-dose regimen of the Astrazeneca, Pfizer or Moderna COVID-19 vaccine. 

Researchers then compared the vaccine effectiveness of those who received a Pfizer or Moderna booster between reported cases of omicron and delta.

In all cases, data showed that the vaccines’ effectiveness against omicron was lower compared with their effectiveness against delta. The effectiveness of both the Moderna and Pfizer boosters was also found to decrease within weeks of people receiving the extra dose.

“Among those who received an AstraZeneca primary course, vaccine effectiveness was around 60% 2 to 4 weeks after either a Pfizer or Moderna booster, then dropped to 35% with a Pfizer booster and 45% with a Moderna booster by 10 weeks after the booster,” the report said.

Those percentages were a far cry from the vaccine effectiveness for individuals who had delta.

Vaccine effectiveness was between 80 and 100 percent in the first week for people who received either Pfizer or Moderna booster, and had only slightly dipped to just over 80 percent by the 10th week for those with the Pfizer booster. 

In the 10th week, for individuals who received the Moderna booster and had delta, vaccine effectiveness was closer to 100 percent.

Researchers also compared the effectiveness of the Pfizer and Moderna boosters for individuals who received an initial two-dose regimen of the Pfizer vaccine.

“Among those who received a Pfizer primary course, vaccine effectiveness was around 70% after a Pfizer booster, dropping to 45% after 10-plus weeks and stayed around 70 to 75% after a Moderna booster up to 9 weeks after booster,” the report said of that data.

“These results should be interpreted with caution due to the low counts and the possible biases related to the populations with highest exposure to Omicron (including travellers and their close contacts) which cannot fully be accounted for,” the report noted.

The report also said, “There are insufficient severe cases of Omicron as yet to analyse vaccine effectiveness against hospitalisation, but this is expected to be better sustained, for both primary and booster doses.”

The CEO of Pfizer said earlier this month in an interview with CNBC that it was possible that people would need to receive a fourth dose of the COVID-19 vaccine in light of the omicron variant.