How the Pfizer vaccine protects us

26 August 2021

By Dr Mark Thomas, Associate Professor in Infectious Diseases, University of Auckland

As of 26 August 2021 New Zealand had administered at least one dose of the Pfizer (BNT162b2) vaccine to 2,012,766 people, of whom 1,102,158 have received a second dose. Vaccine uptake has increased in recent weeks and the rate of vaccine administration appears to closely reflect the volume of vaccine doses received from Pfizer. (1) (Figure 1)

Figure 1. Cumulative number of SARS-CoV-2 vaccinations delivered each week in New Zealand.  The blue dashed line (“Production plan”) indicates the number of vaccination doses planned by the MoH with DHBs and national providers. The yellow line (“Actual”) indicates the number of vaccine doses administered. The green line (“Stock received”) indicates the number of vaccine doses received in NZ from Pfizer. (1)

Vaccine efficacy versus symptomatic disease
A randomised, placebo-controlled efficacy trial of the Pfizer vaccine, conducted in the US and five other nations during 2020, enrolled 43,448 participants; 21,720 who received the Pfizer vaccine, and 21,728 who received a placebo. Symptomatic COVID-19 disease, defined as one or more of: fever, new or increased cough, shortness of breath, muscle pain, loss of taste or smell, chills, sore throat, etc, with onset at least seven days after the second vaccine dose, occurred in seven vaccine recipients and 162 placebo recipients. Vaccine efficacy, against symptomatic disease, was 95 percent. (2) (Figure 2.)

Figure 2. Efficacy of the Pfizer (BNT162b2) vaccine in 21,669 vaccine recipients compared with 21,686 placebo recipients. The first vaccine dose was given at day 0 and the second dose at day 21. Note the clear evidence of efficacy from approximately 11 days after the first vaccine dose. (2)

The vaccine is highly effective against asymptomatic infection, symptomatic disease, disease requiring hospitalisation, severe disease, and death

Vaccination, exclusively with the Pfizer vaccine, commenced in Israel on 20 December 2020, and by 3 April 2021 more than 10 million doses had been administered to more than 5.2 million people. Overall, by 3 April 2021, 72 percent of people aged ≥ 16 years, and 90 percent of people aged ≥ 65 years, had received two vaccine doses. Vaccine effectiveness, in this “real world” situation, was 92 percent against asymptomatic infection, but 97 percentagainst symptomatic disease, regardless of disease severity. (3) (Table 1)

Table 1. Estimated effectiveness of two doses of the Pfizer vaccine, at 7 days after the second vaccine dose, in Israel (24 Jan to 3 April 2021). (3)

A subsequent smaller study, at a large medical centre in Israel, found that SARS-CoV-2 infection occurred in 0.5 percent (27/5372) of fully vaccinated health care workers and in 8 percent (55/696) of unvaccinated health care workers. The relative risk of symptomatic infection in unvaccinated workers was 32 times higher than in vaccinated workers, and the relative risk of asymptomatic infection in unvaccinated workers was six times higher than in vaccinated workers. (4)

Lower concentrations of virus in those vaccinated people who do develop infection
The concentration of virus in nasal swabs, collected from patients with SARS-CoV-2 infection, at a large Israeli health care organisation, was measured to determine whether virus concentration was affected by prior vaccination. The concentration of virus was compared between samples collected from: patients who had not been vaccinated, patients who had received a single dose of the vaccine less than 12 days previously, patients who had received a single dose of the vaccine more than 12 days previously, and patients who had received two doses of the vaccine. When compared with unvaccinated people and people who had received their first vaccine dose less than 12 days previously, the concentration of virus in nasal mucous was significantly lower in people who had received their first vaccine dose more than 12 days previously, or who had received two vaccine doses. (5) (Figure 3.)

Figure 3. Cycle threshold (Ct) for detection of SARS-CoV-2 genes in nasal mucous samples in relation to the duration between vaccine dose and collection of the nasal swab sample. Patients whose nasal sample was collected less than 12 days after their first vaccine dose had a lower Ct value (indicating a higher viral load) than patients whose sample was collected more than 12 days after their first vaccine dose. Patients received their second vaccine dose on day 21, indicated by the vertical dashed line. (5)

Does vaccine-induced immunity wane in the months after vaccination?
A number of “real world” studies have provided evidence that vaccine induced immunity does decline within months of receiving the second dose of vaccine. A large study of all patients diagnosed with PCR positive infection in the UK since December 2020 clearly shows that protection waned within three months of receipt of the second dose. (6) (Figure 4).

Figure 4. Vaccine induced protection against symptomatic Covid-19 disease steadily waned, from 14 days after the second dose, as shown by 19,661 residents of the UK who tested PCR positive between 1 December 2020 and 1 August 2021. The degree of protection provided by two doses of the Pfizer vaccine (BNT162b2) was approximately 94 percent at the beginning of the follow-up period, and approximately 70% three months later. The protection provided by the Oxford vaccine (ChAdOx1) was less throughout the study period but declined more slowly. (6)

Together these studies show that the Pfizer vaccine provides protection not just against disease and death from COVID-19, but also provides a high degree of protection against asymptomatic infection, and also reduces the concentration of virus in the small number of people who do become infected despite having been vaccinated.

Receiving the vaccine provides the vaccinated person with a high degree of protection against illness but also greatly reduces the chance of the person transmitting infection to others. Being vaccinated is good for oneself, and also good for one’s whānau, colleagues and neighbours

More information is needed to determine the best approach to maintaining a high degree of long-term immunity.


  1. NZ Ministry of Health. COVID-19: Vaccine data.
  2. Polack FP, et al. Safety and efficacy of the BNT162b2 mRNA Covid-19 vaccine. New England Journal of Medicine 2020; 383: 2603-15.
  3. Haas EJ, et al. Impact and effectiveness of mRNA BNT162b2 vaccine against SARS-CoV-2 infections and COVID-19 cases, hospitalisations, and deaths following a nationwide vaccination campaign in /Israel: an observational study using national surveillance data. Lancet 2021; 397: 1819-29.
  4. Angel Y, et al. Association between vaccination with BNT162b2 and incidence of symptomatic and asymptomatic SARS-CoV-2 infections among health care workers. Journal of the American Medical Association 2021; 352: 2457-65.
  5. Levine-Tiefenbrum M, et al. Initial report of decreased SARS-CoV-2 viral load after inoculation with the BNT162b2 vaccine. Nature Medicine 2021;27: 790-2.
  6. Pouwels KB, et al. Impact of Delta on viral burden and vaccine effectiveness against new SARS-CoV-2 infections in the UK. MedRxiv 24 August 2021.