Gov. Andrew Cuomo 06102020
Gov. Cuomo: We've been through hell. We'll come out stronger.
02:01 - Source: State of New York

Editor’s Note: This is the first in a series of regular columns on making choices in the era of Covid-19 by Erin Bromage, an associate professor of biology at the University of Massachusetts Dartmouth. His research focuses on the evolution of the immune system and how animals defend themselves from infection. Follow him on Twitter @ErinBromage. The views expressed are his own. View more opinion articles on CNN.

CNN  — 

Every day we are bombarded with new numbers – 20,000 new infections and 800-1,000 new deaths every day; projections such as the total US death toll reaching 130,000 by July 4.

Our responses to hearing these numbers seem to vary widely – some of us lock ourselves away in our homes, while others tell ourselves the threat of infection is over.

Erin Bromage

Rather than running to one of these extremes, it would be better for people to understand the risk and to make choices based on information. But with so much news and data hitting us from so many different sources, how do we make good choices for ourselves and our families?

Most people would look at the number of new cases per day and think this would be a good metric, but it’s not. Cases per day can be manipulated by the number of tests processed per day. When states test more often, they find more infected people. And testing more every day is a good thing.

Why testing matters

Every person who tests positive to the virus can be subsequently isolated, which breaks a transmission chain. You want to see your state testing more, so let’s not crucify states that are increasing their testing when they report the inevitable result of an increase in confirmed cases – especially because the alternative is far worse.

If your state tests fewer people, then the true dynamics of what is going on will be hidden.

The key numbers to watch are how many of the daily tests come back positive, as well as the weekly trend. Are the weekly numbers rising, staying flat, or falling? In a state where anyone can be tested, we want fewer than 2% of all tests to be positive.

In states where they are selectively testing and testing hotspots, then we want the number to be below 5%. In both cases, the lower the number the better the state is doing at responding to the pandemic; a decline in the percentage of positive tests also is a good sign. Nationally, the percentage of tests that come back positive is about 4.5% and flat, but this number varies greatly depending on the state and county.

Currently, 16 states and Puerto Rico are above 5% of testing positivity and in many of those states the trend in percentage of positive tests is going up. One of the most concerning states is Texas. Daily cases are increasing, testing is flat, and the number of positive tests is high (9.7%) and increasing.

This may signify that they are testing largely in hotspots, but the overall trend is not good. In contrast, the early hard-hit states like New York, Massachusetts, and Michigan are all doing well and improving. We know this because the percentage of tests that come back positive and the daily number of new infections is falling.

You can check how your state is doing here.

The truth about herd immunity

One of the questions I am often asked is about herd immunity

Many of you may have heard that the pandemic will be over when we achieve herd immunity. For herd immunity to protect us, however, we need to get between 60% to 80% of the US population immune to this virus. If we reach 80% of the population being immune, the virus would have a very hard time finding a new susceptible host to infect.

But immunity can only occur by natural infection or vaccination, and there is no guarantee that immunity from natural infection will be long-lasting. Only time will tell.

Even in the hardest-hit states, such as New York, only 20% of the residents of New York City have been exposed and successfully fought off the infection, compared to only 2.6% of the people in the Finger Lakes region of the state. This means that the city would need another two to three outbreaks, similar in magnitude to the one in April, to achieve herd immunity through natural exposure. The rest of the state would need years to get to this same goal.

Epidemiologists believe that only 2-5% of the US population nationally has been exposed to the virus and recovered, and infections have not been distributed evenly throughout the population.

This puts us a long way from beating this virus through natural exposure and the prospect of a successful vaccine remains uncertain. Therefore, we must find a way to live with this virus without sacrificing 1% of our population or going back to the strict restrictions of March and April.

However, a second wave of high infection rates and similar mortality rates is avoidable.

What does this mean for your summer?

Our choices over the coming months will determine the trajectory of this pandemic. If we continue to pursue activities that pose a high risk for infection, such as large indoor gatherings, then we will hear the roar of that second wave sooner than later.

If we take a more measured approach, by improving hand hygiene, limiting daily interactions with other people, maintaining physical distance and increasing face mask use when we can’t maintain the distance, then businesses can operate safely, people can return to work and the activities our children are missing can resume.

Our goals must be to lower the risks of infection and reduce harm.

Ultimately, we must all get used to continually tuning our behavior based on local, county and statewide trends in the data. If I were living in the beautiful state of Montana, where there is only a handful of new cases per day, where they are testing well, and only 0.4% of their tests are positive and that trend is decreasing, I would feel much more comfortable eating inside a restaurant and socializing indoor with friends.

On the other hand, if I was living in a county where the test-to-positive ratio was at 10%, limiting daily interactions, physical distancing and mask use would become critical to lower the risk of infection.

Even at Montana’s envious numbers (0.4%), a gathering of 250 people would mean that at least one of those people in attendance would be infected and could infect others. And in another state, the thought of putting 19,000 people into an indoor venue for a rally, when the infection rates are already much higher than Montana, certainly has my attention.

Get our free weekly newsletter

  • Sign up for CNN Opinion’s newsletter.
  • Join us on Twitter and Facebook

    Despite the threat of infection, there are many things I am looking forward to this summer: my first meal at a restaurant (dining outdoors), visiting with more than one or two households at a time, and spending time at the beach. These interactions will be a little different than last summer.

    We will have to keep personal risks and risk mitigation measures in mind, but these adjustments are well worth the payoff of getting to enjoy some of my family’s usual summertime activities.

    We all have a responsibility to reduce harm, not just to ourselves, but also to the greater community. Our actions, and the choices we make over the coming weeks and months, will dictate what comes next and, dare I suggest, even decide whether and when we have a second wave.