COVID-19 Vaccines and Immunity:
Why One Shot Isn’t Enough

As many of us line up for our first and second COVID-19 shots in India, here’s the science and regulations behind the two-shot vaccine.

Why Mustn’t I Miss The Second Shot?

The vaccines currently approved in India and a majority of those developed globally are two-shot vaccines – you get adequate protection only after both doses. The manufacturers make this recommendation based on vaccine efficacy trials. The exception is the J&J shot which is a one-dose vaccine.

Incidentally, many of the other childhood vaccines we take (diphtheria, tetanus, hepatitis B) require multiple doses (2 to even 4) to be effective.

Depending on which vaccine you have taken, the government’s Co-WIN app/platform will open up slots for your second shot once you are within the eligible window.

How Does The Two-Shot Vaccine Work?

The first dose primes or readies your immune system while the second one boosts your immune response, improving its quality and durability. You thus get better protection against severe illness, hospitalization, and death.

You are considered “fully vaccinated” only 2 weeks after both doses of a two-shot vaccine (or 2 weeks after a single shot vaccine like J&J).

How Is The Interval Between Doses Decided?

With Covishield, in trials when doses were given 1 month apart, the vaccine was about 60 to 70% effective. However, when the gap was increased to 2 to 3 months, that efficacy went as high as 90% in trials. This prompted the Indian government as well as others to modify the timing of the second shot for Covishield.

For Covaxin, the efficacy rate in trials was 81% after two shots. No trial data is available on a longer intervals of gap and the recommended duration between shots stands at 4-6 weeks. The stipulated gap for Sputnik V is 3 weeks.

Do I Have Any Protection After One Shot?

You will have some degree of protection against severe illness and hospitalization after one shot. But for maximum possible immunity, the full course will be needed. Simply put, without a second dose the protection may be lower and will not last as long.

Also, the vaccine takes a while to take effect, so it may be anywhere between 2 to 4 weeks before your immune system has reacted adequately to the first shot. So, continue to be cautious.