Taking about an incident this year, a man in Assam's Cachar district reportedly complained to the Deputy Commissioner that he and his wife missed their second dose of Covaxin. In his complaint, the 56-year-old man blamed vaccine shortage for missing the second dose. He also sought an explanation on who will be responsible if something happens to a citizen due to a delay in providing the second shot.

People are concerned about missing their second dose of Covid-19 vaccines due to the shortage in several states. The vaccines presently available in India - Covaxin manufactured by Bharat Biotech and Covishield produced by Serum Institute of India, have varying dosage intervals.

As per data, the government has increased the interval between two Covishield doses to 12 weeks, Covaxin doses need to be taken 28 days apart. But, there has been reasonable concern among people who took the first dose and are missing the second. So what will happen if you miss your second dose beyond this period?

How much gap should be maintained between both doses?

The gap between the two doses depends on the vaccines. The time intervals between two doses are set according to the time for the immune system to generate antibodies with an increased affinity for the antigen. As per experts, each vaccine has its own ideal time between doses.

The preferred gap between doses of the Covishield vaccine is 6 to 8 weeks. The gap between doses of Covishield can go up to 12 weeks as well. Covaxin's second dose can be taken after 28 days.

says virologist Dr. Jacob John, a former virologist at ICMR Centre for Advanced Research, says that the consequence of missing out on the second dose is that you are not protected. “It means that if you get exposed to the infection, you may get infected, get the disease, and even get it severely,” he said.

According to an interim analysis of its phase 3 trials, Covaxin was reported with 78% efficacy, with Bharat Biotech claiming that the efficacy against severe COVID-19 disease is 100%. Covishield too offers 100% protection against severe disease, cases of hospitalization, and death. However, this will happen only after both doses of the vaccines are administered.

How efficient is the single dose?

There’s currently no clarity on the efficacy following one dose of the vaccine. Dr. Aju Mathew, an epidemiologist, says “There is no data or evidence to give us actual estimates of the immunogenicity of one dose of vaccine after the prolonged interval. In clinical trials, the participants got the second dose four to twelve weeks later.”

He says that one can only hypothesize that the impact of a single dose may not be as robust as a person who got a second dose. But it’s still better than someone who is unvaccinated. A significant decrease in mortality rate was seen with the vaccination drive. Many people including the elderly had taken only the first dose when the second wave of the pandemic set in. But what we have noticed is, though infected, no one who took the first dose developed severe COVID-19.

“That is why it is important for all the countries of the world to cooperate and to ensure that every part of the world gets access to vaccines and encourage vaccine acceptance in societies as well. This may repeat a year later. The quicker we could vaccinate the whole world the better we would all be,” he added.

The second dose is supposed to augment the response of the first one and increase the duration of its efficiency. For example, if someone does not have a very good immune response to the first dose, the second dose will take care of it.

Why two doses are needed for immunity?

Both doses of the vaccine play different roles to create immunity against Covid-19. The first dose helps in building a sufficient immune response in the body. The second dose helps in strengthening the immune response and generates memory-B cells which remember the response.

However, the immune response from the first shot will not be "forgotten" by the immune system too soon.

As per medical experts, the immunological memory of the immune system can last about a year.

The peak immunity usually takes up to 14 days after receiving the second dose & only then one is considered safe from Coronavirus.

What happens if you exceed the gap?

The best practice is to follow the ideal gap recommended by each vaccine manufacturer. This gap is set for the best antibody response.

But there are no side effects even if one exceeds the recommended interval, not even mild. As per the experts, if the interval exceeds by over 2 weeks, then the antibody response might not be as strong as expected.

“Single dose does not induce strong immunity. More specifically, if the first dose induces good immunity in a certain percentage of people – say 30% -- for the rest of them, what it does is only priming,” Dr. Jacob says.

Experts say that this does not mean that you need to start the vaccination cycle again. The first dose of the vaccine will have an impact on the body, but people should try to get the second shot as soon as possible.

What is priming?

To understand priming, Dr. Jacob explains the difference between a live virus vaccine-like measles or mumps vaccine and a non-infectious, non-replicating vaccine like the novel coronavirus vaccine.

Unlike a live virus vaccine which multiplies in the body to give you immunity, the non-replicating vaccine works only to prime the immune system. It gradually gets degraded by the body’s physiology.

“One dose of live virus vaccine is more closely mimicking the original infection,” he says. On the other hand, one dose of the non-replicating vaccine is only priming the system. You need to boost it. So it’s called prime-boost immunization. So the second dose can boost the primed immune system.

Even if you increase the potency of the first dose, and achieve better efficacy, it’s still a part of priming. Partial immunity is gained, which will wane over time unless boosted. So in case, you miss the boosting, then you remain vulnerable because you have only been primed.

Why does Covaxin have a shorter gap interval?

Covaxin doses are supposed to be administered within a 6-week gap. Some experts believe that Covaxin may not trigger a strong antibody response with the first dose.

Thus a second dose is needed sooner to mount immunity. Irrespective of the claims and speculations, the fact is that only after two doses can one be marked as safe from Coronavirus.

How late can you take the second dose?

If you miss the dose due to vaccine shortage, try and schedule a date as soon as possible. Getting both shots will help you attain high immunity and minimize the chances of contracting Covid-19.

If you get infected with Covid-19 after getting the first dose, then experts advise waiting for 2-3 months to get the second dose.

This is advised as the immune response due to the infection will provide the body with a natural immunity that may last longer.

Conclusion

Some virologists believe that theoretically, one may need to start all over if the second dose is missed during the recommended time period like they didn’t receive any vaccination at all. You can wait up to three months after the first dose without losing the effect of the first (in the case of Covishield).

Hence, exceeding the period or missing it is not a great idea when there’s the availability of vaccines at your locality.

Important thing is to import vaccines, ramp up vaccine production, and vaccinate on a war footing. The government should ensure buying and distributing vaccines at no cost to all.

If you’re someone who can afford to get jabbed from private hospitals at a reasonable price then go for it. It’s not a wise choice since vaccinations are provided free of cost at government hospitals. But by doing this, the truly deserving or the ones who can’t afford it will have enough supply free through government hospitals.

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