The three vaccine makers which are closest to widespread distribution, namely, AstraZeneca, Pfizer and Moderna, require two doses of the vaccine to work.
A protective effect starts to develop 12 days after the first dose, but full protection requires a follow up shot, according to the World Health Organisation (WHO).
The second dose is considered to be a booster and can be administered anywhere from three weeks after the first one to as much as three months later.
Most vaccines around the world also require two doses to work, including the measles, mumps and rubella (MMR) vaccine. Studies have shown that people who only receive one dose are four times more likely to catch one of the viruses than those of the two-dose regime.
While additional research is still under way to fully understand longer-term potential protection against the virus, the WHO recommends that the second dose be given no later than six weeks after the first.
In the United Kingdom, independent vaccine advisers from the Joint Committee on Vaccination and Immunisation (JCVI) have suggested that due to a high vaccine efficacy, it is more beneficial for countries to inoculate a greater number of people with a single dose.
While the second dose is still important to provide longer lasting protection, one dose could prevent more deaths and hospitalisations than vaccinating a smaller number of people with two doses.
Why is the second shot necessary?
Vaccines work by teaching the body’s immune system to recognize and mount a defence against a virus.
When a person recovers from Covid-19 without having received a vaccine, their bodies naturally develop antibodies against the virus which may provide future immunity. However, these antibodies have been shown to fade after around three months.
The first dose of the vaccine exposes the body to a modified version of the virus.This should help prime your immune system to attack the coronavirus if it does to enter your body in the future.
The second shot or the booster dose is a way of re-exposing the body to the antigens or the molecules that causes your immune system to produce antibodies against the virus.
According to published results, the Pfizer-BioNTech vaccine was 53% efficient after the first dose and around 90% efficient after the second.
How many doses are needed in South Africa?
South Africa’s population stands at over 59 million, which means the country will need to procure more than 118 million vaccine doses if it is to vaccinate the entire population.
The Health Department aims to vaccinate around 40 million people by the end of the year, equating to around 67% of the population. If the target is met, top scientists say the country would have achieved population immunity.
If 40 million people are to be vaccinated by year-end, this means South Africa would need to acquire at least 80 million doses.