In this article we have presented a new perception of herd immunity threshold (HIT) which considers that only a “band of population” are susceptible to any pathogenic infection. This is termed as the “effective herd immunity threshold” (EHIT) and the progression of the disease (caused by this pathogenic infection) is mainly determined by this EHIT value. We have argued here that this EHIT value (considering the immunity band picture in the population) will be substantially lower than the estimated canonical HIT values obtained from various existing models. We propose that the actual prediction of the disease progression should now be calculated using the EHIT values.

We have analysed the death and recovery rate of Covid-19 disease progression. From the analysis, we have argued that the pandemic is over in certain countries (labelled as group-A) and for other countries (labelled as group-B) the disease appears to remain as endemic. Taking into account the serological survey (sero-survey) test results obtained by certain groups and comparing it with herd immunity threshold value one can infer that the low number of infection for group-B is either due to acquired immunity by some previous infection by other coronavirus or due to innate immunity towards this infection. This effect is stronger for group-B to slow the progress of the disease to such an extent resulting in flattening of the disease progression curve compared to group-A.

We have proposed a model considering two equally sized population (group A and group B) with low and high levels of disease tolerance. We have argued that in the more tolerant group (group B) the progression of the disease with respect to time will be slow with lower number of infections at any given time. We attribute this effect to the innate immunity which advantageously, can also be one of the major contributing factors for flattening the curve. We have compared the growth of Covid-19 disease in various countries to understand this effect.