Booster dose effective in reducing COVID-19 across ages in adults

The risk of COVID-19 is substantially reduced in adults who receive a third “booster” dose of the BNT162b2 vaccine regardless of their age, according to a study from Israel.

“We … found that the booster dose reduced the rate of confirmed infection and severe illness by a similar factor in the age groups studied,” the authors said.

The Israel Ministry of Health database was utilized to identify individuals aged ≥16 years who, between July 30 and October 10, 2021, had received two doses of the BNT162b2 vaccine ≥5 months earlier (n=4,696,865). Individuals who had a PCR-confirmed SARS-CoV-2 infection prior to booster eligibility were excluded. Each individual who had received a booster ≥12 days prior (booster group) was matched with an individual with a similar risk profile who had not yet received a booster dose (non-booster group).

A total of 83,481 confirmed infections were documented in the non-booster group, 6,160 in the booster group, and 8,880 in the early booster* group. In terms of severe illness, 1,171 cases were documented in the non-booster group, 175 in the booster group, and 136 in the early booster group. A total of 298 deaths occurred in the non-booster group, while 35 and 46 deaths were documented in the booster and early booster groups, respectively.

Confirmed infection rates were lower in the booster vs non-booster groups across age groups (rate ratios [RRs], 12.3, 12.2, 9.7, 9.0, and 17.2 for age ≥60, 50–59, 40–49, 30–39, and 16–29 years, respectively). This corresponded to adjusted difference rates of 57.0, 69.0, 81.7, 89.5, and 72.2 infections per 100,000 person-days between groups in those respective age groups. [N Engl J Med 2021;doi:10.1056/NEJMoa2115926]

Confirmed infection rates were also lower in the booster vs early booster group across the age groups (RRs, 7.4, 7.2, 5.4, 4.9, and 10.8 for age ≥60, 50–59, 40–49, 30–39, and 16–29 years, respectively), corresponding to 34.4, 38.3, 38.2, 36.9, and 35.7 per 100,000 person-days, respectively.

Two age groups were assessed for COVID-19 disease severity** (age ≥60 years and 40–59 years). Severe illness rates were lower in the booster vs non-booster groups for both these age groups (RRs, 17.9 and 21.7, respectively), corresponding to adjusted difference rates of 5.4 and 0.6 per 100,000 person-days, respectively. However, the confidence intervals were wider in those aged 40–59 years due to fewer cases in this age group. Similarly, lower rates of severe illness were observed in the booster vs early booster groups in both age groups (RRs, 6.5 and 3.7, respectively), corresponding to adjusted difference rates of 1.9 and 0.1 per 100,000 person-days, respectively.

The rates of severe disease among those aged 16–29 and 30–39 years were very low, the authors said.

COVID-19–associated death rate among those aged ≥60 years was also lower among those in the booster vs non-booster group (RR, 14.7), with an adjusted rate difference of 2.1 per 100,000 person-days, and lower in the booster vs early booster group (RR, 4.9; adjusted rate difference, 0.8 per 100,000 person-days).

The authors acknowledged that despite efforts to reduce confounding, they could not account for certain factors such as risk-avoidance behaviours and comorbid conditions. They also suggested that behavioural biases may have led to underestimated infection rates. For example, individuals may have undergone fewer PCR tests in the initial days after receiving a booster, thus reducing the likelihood of detecting infection.

Additionally, the timing of the study suggests the results apply to the delta variant. As such, further research is necessary to determine the long-term effectiveness of the booster dose against other variants.