Both the virologists and immunologists have been questioning the idea of herd immunity against the coronavirus at the moment.
Following to the infection or vaccination, immunity against the coronavirus may start to wane. Impact of the coronavirus variant may play an important role on the occasion also. Therefore, requirement of vaccination may be seen every year. It is possible to observe cases every year.

Prof Miles Davenport, program head of Kirby Institure from the infection analytics program of the University of New South Wales in Sydney has said “the concept of herd immunity is that once it’s achieved, then you will not have circulation of virus in the community”
Later on, he has also added “I think that many scientists in immunology or virology are strongly questioning whether herd immunity is possible at this stage. It’s going to continue to circulate.”
Herd immunity concept is effective when a large number of population has been infected or vaccinated against the disease then antibodies may be developed within the body. Infected person may have lesser chance of infecting others on the occasion. As a result, spread of coronavirus can be reduced.
Percentage of the population that is needed to be infected or vaccinated against the coronavirus depends on the nature of spread. However, estimated percentages on the occasion are about 60% to 70%. Herd immunity can be achieved with this percentage.

According to the preprint paper that has been published by Davenport and his colleagues from Kirby Institute has said that diminishing immunity and new variants can affect the protection level of vaccines. Therefore, booster vaccine may be required. Vaccines may not be enough to prevent the infection. However, it may be good enough to reduce the severe level of illness.
Davenport has said “Herd immunity was always going to be a high hurdle for vaccination. We don’t expect that from the flu vaccine for example. Like flu, the major risk of infection seems to come from your first exposure to the virus. If vaccination can achieve a baseline level of immunity, it will hopefully provide enough protection to prevent severe disease in vaccinated people – even if virus continues to circulate causing mild infection. This is essentially what we see with flu and the other ‘common cold’ coronaviruses.”

Later on, he has also added “The question is, does the virus continue to circulate in a very mild form so that you no longer need to give vaccine boosters because this very mild infection provides that boost?”
Right now, four strains of coronavirus have been circulating all over the world. Long term immunity cannot be earned after the vaccination. So, people can be infected by the same strain every year.
If herd immunity can be achieved then it may mean that you can open the border without any restrictions. However, herd immunity is really a concept. So, the result cannot be determined accurately.