Covid-19 vaccine: data-backed answers to pressing questions

Social media is bursting with discussions around the vaccine and unfortunately these discussions are often full of misinformation. Here, I’m reviewing the science behind the Pfizer vaccine and answering some basic but important questions.

How does it protect you? And are we sure it works?

Contrary to opinions I’ve seen recited in social media, the vaccine doesn’t simply reduce the severity of the symptoms, it reduces the chances you will develop COVID-19. In response to the vaccine your body develops antibodies that can fight off the virus early, if infected. The figure below, from the first phase of the trials, shows the amount of antibodies to SARS-CoV-2 before vaccination and after first and second vaccine doses. The amount of antibodies after vaccinations is obviously much larger than in the placebo group and before the first vaccine dose. You can also see that the second vaccine dose increases the amount of antibodies and that the amount of antibodies after the second dose exceeds the amount of antibodies that result from an actual SARS-CoV-2 infection.

Antibodies developed in response to the vaccine from Walsh et al. 2020. Please note this image was modified from the original version published in Walsh et al. 2020. I’m showing only one variant of the vaccine that was tested, for brevity. This variant was BNT162b1, 10 mg, tested on participants 18-55 y.o.

Each bar shows the amount of SARS-CoV-2 antibodies for (left to right) 1) Placebo group (gray dots), 2) right before the first vaccine dose (1 with black arrow), 3) right before the second vaccine dose (21 with black arrow) and 4) 28 days after the first dose (7 days after the second dose) 5) 35 days after the first dose (14 days after the second dose) and finally 6) the same antibodies in a group of people who were previously sick with COVID-19 (labeled HCS, black dots)

In the second and third phases of the trial, the main outcome measure was the number of people developing COVID-19 after the second dose of the vaccine, as confirmed by symptoms and a COVID-19 test. The study showed that out of 18,198 people who received the vaccine, only 8 developed COVID-19 (0.04%) at least 7 days after the second dose, while out of the 18,325 people who received the placebo 162 developed COVID-19 (0.9%). Note though that they did not measure asymptomatic transmission, so it is possible that vaccinated people still carried the virus around without symptoms. The 95% efficiency reported for this vaccine is derived by the following formula – (1 – 0.04/0.9)*100 = 95%.

Cases of COVID-19 in the placebo and the vaccinated groups from Polack et al. 2020. Please note the original figure was modified here by adding labels for clarity.

The figure shows cumulative cases of COVID-19 (as % of people in each group) in the placebo group (blue line, square markers) and the vaccinated group (red line, round markers). Cases in both groups increase similarly until about ~12 days after the first dose. Then the vaccinated group shows a flattening of the curve, while the placebo group continuous to increase at the same rate. In the vaccinated group the incidence of cases slows down even more 7 days after the administration of the second dose (28 days on the x-axis).

Who did they test the vaccine on?

The vaccine was tested on people from 4 different countries (USA, Brazil, Argentina and South Africa). About half of the sample were women, 58% were between 16-55 y.o. and 42% were 55-85 y.o.. There were 83% whites, 9% Blacks, 4% Asians. 28% of the sample were Hispanic. 35% of the sample were considered obese.

People who had preexisting conditions constituted ~20% of the sample. Vast majority of these people either had chronic pulmonary disease (e.g. bronchitis) or/and diabetes. Others had HIV, specific types of cancer, myocardial infarction, cerebrovascular disease, rheumatic disease and others. For a full list please see Table S2 in the Appendix to Polack et al. 2020.

Who did they NOT test the vaccine on?

The trial excluded pregnant and breastfeeding women, children, people who experienced severe reactions to vaccines or severe allergic reactions to any of the COVID-19 vaccine components in the past, immunocompromised / immunodeficient people, people who receive immunpsuppressing therapies and potentially others (see section Exclusion criteria in the Protocol amendment 9 for details).

Is it safe to take?

Safety has been assessed in the 2-3 phases of the trials in 2 ways: 1) solicited detailed reports from a subset of the placebo and vaccine groups on any adverse events within 7 days after vaccination using an electronic diary and 2) unsolicited reporting of adverse events from anyone in the two groups up to 1 month post 2nd dose and serious adverse events up to 6 months after the 2nd dose.

The figure below shows the incidence of various adverse events within the 7 days of vaccination in the subsample that completed the electronic diaries (8,183 participants). The most common local reaction was mild-to-moderate pain at the injection site. Common systemic reactions were fatigue, headache, chills and muscle pain and less frequently fever and joint pain (fatigue and headache were also experienced by the placebo participants). In general, older participants (>55) experienced less adverse events than younger participants.

Adverse events within the first 7 days after vaccination in a subsample of placebo and the vaccinated groups from Polack et al. 2020. Please note the original figure was modified here by only displaying one of the panels for brevity, other panels show reactions from other age groups and dose 1 administration.

How does it compare to the flu vaccine? For the inactivated flue vaccine (dead virus) soreness (pain) is a common local reaction to the flu vaccine (affecting 10–64% of recipients). Fever can also occur, especially in children (5-12% of recipients). For the live attenuated flu vaccine fever and body aches are more common (16-31% and 14% respectively).

How about the rest of the group that were not part of the diary study? The table below, taken as is from Polack et al. 2020, shows the total incidence of adverse effects in the full sample at any point after the 1st dose of the vaccine. As can be seen in the table there are more adverse effects in the vaccinated group, but it is primarily due to the incidence of transient local and systemic events shown in the graph above.

What don’t we know about the vaccine?

There are a few things we don’t yet know about the vaccine either because of the limitations of the study and/or because the vaccine was developed and deployed so quickly:

  • The current papers don’t have data on asymptomatic transmission in the vaccinated group. In other words we don’t know how many people had asymptomatic Covid-19. The scientists are planning a follow-up to answer this question.
  • Adverse events were only recorded up to 2 months post the vaccine. We don’t know if there are longer term adverse effects.
  • We also don’t know how long the protection from the vaccine will last
  • Finally, we don’t know how safe and effective the vaccine is for the populations it was not tested on – children, pregnant women, people taking immunosuppressants

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