COVID outcome may highly depend on viral exposure intensity.

For a specific person, a virus infection tends to be thought to have a predetermined outcome.  For example, a young healthy person would be asymptomatic after Covid infection, and an old and sick person would die or be hospitalized.  This explains most observations generally speaking. 

Since the start of the Covid 19 pandemic,  a tremendous amount of data has been accumulated easing the verification of our hypotheses. The aforementioned all-or-none model has difficulty in explaining some data.

For example, both New York City and South Dakota experienced large Covid waves in 2020 albeit at different times.  They likely have 30-40% of their population infected. South Dakota's excess mortality (compared with 2019) is 23%, but the NYC's is 53%.  Demographic difference unlikely can fully explain this difference.

The current north-south divide in the US in terms the ▲ infection cannot be attributed to vaccination rate difference alone because Florida has a higher vaccination rate than North Dakota though most of the hospitalized and dead are unvaccinated.  

Sweden, which adopts a herd immunity strategy, has an excess mortality rate of 5.2%, much lower than Spain (14.3%), France (10%), Belgium (10.5), Switzerland (11.5) and many other European countries.  Excess mortality is the best way to assess Covid mortality because the reported Covid deaths can be controversial. 

We can explain all of them with the assumption that COVID outcome depends on viral exposure intensity that is defined as ambient viral load multiplied by exposure time.

Let us use a hypothetic scenario to explain this. Suppose one breathes in 10 copies of coronavirus every hour indoors and they enter his body through mucous membranes. He es exposed to the virus for only 1 hour in a store one day.  When these copies start to replicate, the race between them and the immune system starts, and the immune system beats the virus this time and the person is asymptomatic or has mild symptoms.  This infection is very similar to vaccination.

Now, suppose another person stays in a confined space (e.g., an apartment building) constantly and also breathes in 10 copies of the virus every hour,  so 240 copies of the virus enter his body/day.  Due to the much larger number, the virus may win the race against the immune system causing severe symptoms leading to hospitalization and death.

The above difference is exactly the difference between the north and the south.  People in the north spend most of their time outdoors or indoors with windows/doors open with shorter exposure to the coronavirus at a lower load.  Most people in the south spend most of their time in closed air-conditioned buildings with circulated air without virucidal filtering. 

This also explains why lockdowns are counterproductive by forcing people to spend more time indoors when the virus is widely spread in a free country.  NYC experienced the worst Covid hit in the US last year due to a triple whammy: 1. busy Covid incubators - subways; 2. high population density and high portion of people with shared indoor space. 3. Lockdown to force people to stay indoors.  This explains the difference between NYC and South Dakota alluded to earlier.   

The counterproductive lockdown also explains the current Vietnamese situation.  They will be able to bring down the Covid infection after paying an unnecessarily high price.

Sweden has never restricted outdoor activities. Their population likely has been widely infected but in a low viral load manner, similar to vaccination.  This may explain its low excess mortality. 

Since many institutions have failed to provide effective ways to combat Covid, and a large number of grossly misinformed people make any effective collective effort impossible, we have to take it upon ourselves to protect ourselves and our loved ones. The following are effective measures without affecting enjoying a normal life:

1. Vaccination.  It reduces hospitalization/mortality chance by 20 times or more.
2. Maximize outdoor activities and minimize indoor activities.
3. Open windows or doors if comfortable. 
4. Install an invisible UVC air sanitizer in the HVAC.  If this is not feasible, the easiest way to reduce ambient virus load is using a UVC light and make it invisible (UVC is harmful to eyes and skins). For example, you can place the UVC light in a room without people and turn it on/off with a remote control (e.g., Google Home on a mobile phone). When ventilation is turned on, air going through that room will be sanitized.
5. Effective masking. Most masking is for protecting others by filtering microdroplets of the wearer.  To protect ourselves, we can wear an N95 mask properly (i.e., tightly). This can be very uncomfortable especially for a long duration. The better alternative is wearing a UV respirator with low airflow impedance. Some UV respirators require a mask over them to make the UV invisible.  You can use a mask with low airflow impedance such as a surgical mask or cloth mask. 
6. If convenient, maintain a comfortable social distance.  Avoid symptomatic people. 

Method 2 to 6 reduce viral exposure intensity.  They enhance the chance of avoiding infection or getting an infection that is so mild that it will effectively be similar to vaccination.