At a seminar about covid immunity and vaccines at JIPMER, attended by experts immunologists Dr Amita Aggarwal (SGPGI),
Dr Andrew Pollard (UK),
Dr N K Arora,
Dr Gagandeep Kang (CMC Vellore) and epidemiologist like Dr Muliyil (formerly CMC Vellore).
the summary of post Covid immunity status is:
1. Immunity following natural Covid infection, based on our experience with previous SARS-CO1 and MERS infections has shown that protective IgG antibodies rise after covid infections.
After 3-4 months, their levels go down to certain level but are usually detectable upto 2 years.
(The antibody level at which there is protection against covid19 infection still remains unknown)
Specific T cell immunity which is equally important has been shown to persist even upto 17 years.
This simply means, in Covid19, possibly recurrence of infection is an exception (very rare), not a rule.
Therefore, till now out of millions of infections, only 41 cases have been reported to have 2nd infection and none of them were severe infections.
We expect vaccine immunity to follow similar rules, as set by natural infection with covid19.
2. Estimates based on R0 of SARSCOV2 says atleast 70% of our population need to get immunity either by natural infection or by vaccination. Until this figure of 70% immunity is crossed, infection is unlikely to stop.
Recent ICMR seroprevalence study from India representing wider population from various parts of the country (before vaccination started) has shown presence of protective antibodies in 35-40% of population in major cities and 10-20% in rural areas.
This immunity has been non uniform, which means pockets of susceptible people are there in cities, more so in villages.
Any time this could lead to surge of cases, like it’s happening now in certain parts of India.
3. Out of >200 variants of covid19 known till now, only 3 have been found to be relevant for spread of infection – the UK, Brazil and South Africa variants.
When serum of those who received covishield, was seen whether it could neutralize these variant viruses, neutralization was ok (~90%) with UK and Brazil variants. However for SA variant it was ~40-50%. Seems like SA variant was about to escape these antibodies in a significant proportion.
But T cell response mounted by vaccine (by stimulating killing of entered virus) is expected to protect againt severe infection (needing hospitalization and/ or death).
Such variant is reponsible for 90-95% of infection in SA.
Its possible that newer variants could arise in future, which could circulate in India also.
It is possible that those who got vaccinated with covishield (mounted immune response) or got immunity by natural infection may get infection by such variants, but they are very unlikely to develop a serious disease (hospitalization and death) in future because of their protective T cell response.
So, vaccinated with covishield or previously infected people could get mild infection by such variants but never a serious disease.
It has been shown that these variants have increased rate of infection, (mainly in unimmuned individuals) but not linked with severe disease.
Covaxin being whole cell vaccine is likely to provide better coverage against these variants.
4. If you see the trend of current rise in cases in India, rate of infection (per day) has increased to 0.4% from 0.1% (which was persisting from last few months). (In Cdg upto 0.9% from 0.1%).
But fortunately rate of death has not increased which persists to 0.1% only, like before, this is reassuring, for the moment.
5. But until we achieve immunity in 70% of our population, such future waves among susceptible population cannt be ruled out.
Such waves are likely to continue atleast for next 1-2 years (eventually viruses are expected becomes less lethal over a period of time).
6. Although current vaccine companies including covishield will be producing newer versions of their vaccines in coming days (which will provide better coverage against these varients), but currently we don’t think, any boosting with them will be an answer.
If a new variant causes severe disease (hospitalization and death) in those who had already achieved immunity in past (which is something not expected at present), then the answer will be to have newer negeneration of vaccines which will provide much wider coverage.
What should be done in the present scenario:
a) After CDCs newer recommendations for vaccinated individuals, completely vaccinated asymptomatic individuals need not be asked to isolate themselves (for 1-2 weeks like before), as risk of asymptomatic infections after vaccination is very less.
Symptomatic exposed should be tested, if negative, no isolation.
b) Although counting positive cases should continue (for epidemiology), but should not worry much.
Worry should be if hospitalization and deaths increases.
c) Possibly we need to think about policy of not isolating those who are covid pcr positive but have CT value more than 28.
This could change the whole scenario.
d) Since target at the moment is not to achieve 70% of immunity in population (unless we could vaccinate whole population, almost an impossible task), focus needs to be to protect the vulnerable from hospitalization and death i.e. above 65 and 50-65 with comorbidities.
Ensure those who are known to you get themselves vaccinated.
We need to learn to live with this, alteast for few more years.
Dr Sanjay Verma,
Professor of paediatrics