We plot three scenarios: Optimistic: vaccine immunity does not reduce Intermediate: vaccine immunity is halved Pessimistic: vaccine immunity goes completely – Mahindra Agarwal
Note that the initially numbers would be very small and so genome sequencing may not throw up any case, especially if (as has been reported) most of the cases are mild and thus may go unreported. So it is not a surprise that first case was reported in November.
Why was the initial spread slow given that it is a highly infectious mutant?
Reach of the pandemic from July until now is around 85% (this is computed using the serosurvey that showed ~47% seropositivity in May ’21). Calibrated model shows that natural immunity in Sep was ~77%.
This means that only ~10% population within reach was available for infection, thus slowing down the spread significantly.
Why are the numbers rising rapidly now?
It is likely due to an increase in reach. The simulation is not converging for the latest phase that started on Nov 19 due to fluctuating data — hopefully will converge soon. We can then state with certainty if reach has increased.
Why would reach start increasing now, four months after beta started increasing?
This is a common phenomenon observed during spread of delta also: reach started increasing in India two months after beta did. Longer gap in SA is, again, due to very small number of susceptible.
A closer look provides different conclusion. Likelihood of infection is proportional to fraction of susceptible population. This fraction has gone down from ~10/85 in Aug (~75% with natural immunity out of ~85% within reach) to ~5/85 now, a two-fold reduction.
Add to this the fact that the study also shows that total number of reinfections until now are about 1/80th of total number of infections. Taking into account that reinfection cases are tested positive both times, the ratio of actual reinfections to actual cases comes to ~1/9.
That is, ~11% were reinfected over ~14 months. Thus, rate of loss of immunity is miniscule, and even an increase by a factor of 1.5 would hardly move the needle. We can conclude that omicron is not bypassing natural immunity in any significant way.
Let us use these inferences to predict the future trajectory in India assuming that omicron has already arrived and is spreading.
Current values of parameters for India are: beta ~ 0.67, rho ~ 95%, natural immunity ~ 83% (calibration used is ICMR serosurvey of June 21).
We assume that:
(i) beta goes up by a factor of two to 1.33 by January
(ii) rho increases to 100% by February
(iii) natural immunity is lost at the rate of 6% per month
It may well be that hospitalization load is even lower as there are indications that the cases are mostly mild. We need to wait for more data to be sure.
Conclusions:
(1) Evidence so far suggests that there will be a mild third wave in India early next year.
(2) As observed during spread of delta, a mild lockdown (night curfew, restrictions on crowding) can bring down beta substantially. That will significantly reduce peak value.
