A new study shows that the RO of SARS-CoV-2 is 5.7 rather than 2.5. This means that a single person infected with SARS-CoV-2 is more likely to infect 5 or 6 other people. RO (R-naught) is the reproductive number of an infectious agent. It shows how many people can catch the disease from a single infected person.

With an RO of 5.7, 82% of people need to be immune to reach herd immunity. RO can help scientists identify the intensity of an outbreak. If a disease has an RO value of less than 1, it will most likely die out. RO greater than 1 means the disease will keep spreading.

Compared to SARS-CoV-2, the RO value of the common flu's virus is of 1.3. This makes COVID-19 a lot more contagious than the flu virus. But a higher RO does not always mean a more dangerous disease. SARS had an RO of 2 to 5 and infected just over 8,000 people.

With a lower RO, seasonal flu infects millions of people every year. The number tells us the potential transmissibility of a virus. It does not actually tell you how fast a disease will spread. Also, the RO does not remain constant. As this is an average, it varies from one place to another.
The actual transmission rate can be lowered through screening or quarantine measures.

A simple hand washing step can lower the viral transmission rate.

RO also depends on how long a person remains contagious. Adults with seasonal flu remain contagious for 7-8 days.

If the infectious period of the disease is longer, it is more likely that the infected person will spread the disease to other people. In the case of COVID-19, we are still not sure about how long the person remains contagious.
When dealing with pandemic health agencies try to bring down the RO down to less than 1 by enforcing measures such as personal hygiene and quarantine. This is tough for diseases like measles that have a high RO.

Bringing down RO is also more difficult in densely populated regions compared to places where people are more spread out. The estimated RO of SARS 2003 pandemic was around 2.75. But this value dropped below 1 after a month or two. This was made possible by tremendous efforts of quarantine activities and isolation.

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