An article recently published in the New England Journal of Medicine reports on the effect of administration of a booster Pfizer vaccine dose in people in Israel, aged more than 60 years, who had completed their initial vaccine course approximately six months previously.
Figure 4. The figure (from “world in data - covid”) shows the successive waves of the SARS-CoV-2 epidemic in Israel from 12 March 2020 to 30 October 2021. The green arrows represent the two doses of the primary vaccination course given 21 days apart, in January-March 2021, and the third single booster dose, given in August 2021.
In response to a large epidemic wave that began in November 2020, the Israeli government delivered a vaccination programme that administered two doses of the Pfizer vaccine to more than 50 percent of the population by the end of March 2021. However, the emergence of the Delta variant and waning vaccine immunity was followed by another large epidemic wave that began in August 2021. In response to this surge in cases the Israeli government approved the delivery of a third dose of the Pfizer vaccine (identical to those used for the two primary vaccinations) for high-risk people and those aged more than 60 years.
The incidence of symptomatic SARS-CoV-2 infection, and of severe COVID-19 disease (defined as resting respiratory rate >30/min, or oxygen saturation <94% while breathing air) was studied in people who had received two vaccine doses before 1 March 2021, and who had, or had not, received a booster vaccine dose between 30 July 2021 and 31 August 2021. The incidence of infection, and of severe COVID-19 was studied for the same 32 day period (30 July 2021 - 31 August 2021). Thus, the effect of the booster dose could not be determined for durations greater than 32 days. Approximately 700,000 study subjects received the booster dose and approximately 400,000 did not. (5)
SARS- CoV-2 infection occurred in 4,439 (of approximately 400,000) people who had not received a booster, and in 934 (of approximately 700,000) people who had received a booster. The rate of infection was 11.3 times lower in the boosted people than in the non-boosted people. Delivering a booster dose to 1,000 people reduced the number who developed infection from approximately 26 to approximately 2.6. The number needed to be vaccinated to prevent one infection was approximately 40.
Severe COVID-19 occurred in 294 (of approximately 400,000) people who had not received a booster, and in 29 (of approximately 700,000) people who had received a booster. The rate of severe disease was 19.5 times lower in the boosted people than in the non-boosted people. Delivering a booster dose to 1000 people reduced the number who developed severe COVID-19 from approximately two to approximately 0.14. The number needed to be vaccinated to prevent one case of severe disease was approximately 500.
Vaccine efficacy against infection, which was estimated to have fallen to approximately 50 percent at 6-7 months after the primary vaccine course, was estimated to again be approximately 95 percent by 12-20 days after the booster dose. The article provided no information on either the numbers of cases requiring hospitalisation or on deaths.
A very recent press release from Pfizer, in mid-October, reported that a third vaccine dose, given approximately 11 months after administration of the second dose, reduced by 95 percent, the chance of having symptomatic SARS-CoV-2 infection. Symptomatic SARS-CoV-2 infection occurred in 109 people, out of approximately 5,000 who received a placebo third dose, but in only five people, out of approximately 5,000 who received a Pfizer vaccine third dose. (6) The number needed to be vaccinated to prevent one infection was approximately 50. Unfortunately the Pfizer press release provided no information about the impact of the booster dose on rates of severe covid-19, hospitalisation or death. The manuscript of the article that reports on this study is not yet available.
In summary, there is clear evidence that a booster dose of the Pfizer vaccine does increase serum neutralising antibody titres, and also reverses the decline in vaccine efficacy against infection, which occurs in the months following administration of the second vaccine dose. However, there is also good evidence that despite vaccine efficacy against infection waning significantly in the months after the primary vaccination course, vaccine efficacy against hospitalisation and against death remains extremely high.
The possible benefits from a booster vaccine dose will differ markedly depending on the individual’s risk of severe disease, hospitalisation, and death, and on the potential consequences of infection. People at particularly high risk of severe disease, or death, such as those over the age of 80 years, and those with severe underlying conditions, may derive benefit from a booster dose, but the duration of this benefit is unknown at present. People who, if infected, might transmit infection to a person at particularly high risk of severe disease may also be suitable candidates for a booster dose, but the same uncertainty about duration of benefit also applies to them. We need more detailed data. However, the greatest need at present is clear, to deliver a primary vaccination course to all those who wish to receive it.