It is used to refer to a significant increase in the number of infected cases after a pandemic has remained suppressed for a number of days.
This is not a scientific-term, so there are no definitions of ‘significant increase’, or ‘how much suppression and for how many days’.
Second waves of various pandemics such as Spanish flu (1918-1920) and H1N1 epidemic (2009-10) were devastating and that is why it has become a cause of fear in the case of coronavirus.
Pandemics are caused by viruses, bacteria or other microorganisms, for which humans haven’t built an immune protection.
Pandemics are rare but most common of them have been influenza-based; influenza is a contagious disease of respiratory tract caused by viruses.
And because viruses are very good at creating variants?E.g. let’s say a virus A attacked a human body; the human body will develop immunity for A so that A no longer poses a risk, but the virus may create a variant A’ next time & still be lethal because immune system hasn’t dealt with A’ before. of themselves, influenza has never been wiped out completely.
But influenza may get beaten temporarily because of change in weather?Coronavirus has not been found to be strongly affected by weather or herd immunity..
Herd immunity is when a sufficient portion?In case of coronavirus, the sufficient portion has been estimated to be between 55% to 82% of a community becomes immune (because of recovery or vaccination) to a variant of a virus and the virus can’t find new hosts.
Until the virus creates an equally dangerous variant of itself, against which the humans are yet to develop immunity.
Lifting of preventive measures before development of herd immunity or the virus’ timely creation of a new, comparably-dangerous variant are a couple of reasons why second waves make a comeback.
